Step 1: Location
First a note on locating the shelter for the most accurate readings: The National Weather Service (NWS) advises that your sheltered thermometer should be
-between 4' to 6' above fairly level ground
-which is typical for your area (sand with a little grass here),
-has good cross-ventilation,
-and which is located 100' or more from concrete or asphalt paving
-and at a distance from the nearest building equal to four times the height of the building. As my house is about 12 feet high, I needed to put the post about 50' from the house. The advertised range for my thermometer is 100', but if you've got a cheap thermometer as I do, I would leave a wide margin for error (i.e. no more than 60' or so).
Now if you live on a small lot in town, you are looking at this and thinking "no way!" This is true if you want to supply data to the NWS. On the other hand, if you just want to know what the temperature is, say, out in your garden, put your shelter there and don't worry too much about it.
Step 2: Materials and Tools
Do not go by these measurements unless you are sure they fit the materials you are using! The post thickness in particular may vary. Also make sure your remote sensor will fit within this design.
2-3" x 16" x 3/4" plywood uprights
1-3" x 9" x 3/4" plywood floor
6-~3/4" x 3/4" x 4" hardwood vent supports
1-3/4" x 7" x 2 5/8" top brace
1-2 5/8" x 1" x 3/4" crosspiece for hanging the thermometer
1-~11 1/2" x 11 1/2" x 3/8" plywood for the ceiling
1-~12" x 12" corrugated or other roofing
1-8' landscape timber (the red ones are cheaper, but much less sturdy and durable. Choose the straightest one you can find.)
2-8" x 16" 3-panel aluminum vents (the official term for these is, apparently, "face on" vents)
4-1/2" long pieces of 3/4" PVC pipe
4-2" bolts, lock washers, and nuts
4-machine screws, lock washers, and nut
9-1/2" screws (1 is to secure the thermometer)
8-1 1/4" deck screws
Titebond II wood glue
folding rule or tape measure
posthole auger or digger
Materials & Tools for Optional Floor (see note in step 8):
2-6" x 9.5" aluminum lawn edging
Step 3: Shape Vent Cover
The size of the shelter structure depends on the aluminum vents you'll use to cover it, so I bent these to shape first. With two 3-paneled vents, I decided on a 6-sided vent cover. If you want a larger shelter, other configurations are possible, although construction will get more complicated.
The vents come with screening attached to the back. This will get in your way, so take it off first. I found that a fork tine poked into the mesh allowed me to pull the screen out without prying up the aluminum lip that holds it in place. A bent piece of wire would also do the job handily.
Bend the vent carefully where the panels join. The edge of a table makes this fairly easy if you work slowly; the aluminum is very thin. Avoid bending the louvered panels. Then bend the two 8" edges of the vent where it will be bolted to the other vent.
Now do the same to the second vent.
Use the machine screws, lock washers, and nuts to bolt the two together. Adjust the bends so your vent cover is as symmetrical as you can get it. Measure the distance between opposite bends to get the outer dimensions of your cover. This will tell you the minimum dimension of the square of plywood you need for the shelter's ceiling. This should be in the neighborhood of 10", but as you want some overhang, make the plywood ceiling at least 11" by 11".
Step 4: Build Framework
The two upright legs will fit over the flat sides of the landscape timber you will use for a post. I made them 2 5/8" apart, which was the width of my particular post, but please measure yours and go by that, as they are not uniform. As my remote sensor was about 2 1/4" wide, this worked fine.
The upper (in the photo) flat piece forms the floor of your shelter, and when finished, sits on top of the post. It should be as long as the distance between the two center vent panels (~9" for me), which will be screwed to it for more stability. The photo showing the vent cover in place should make this a little clearer.
The bottom (in the photo) flat piece is your top brace, joining the uprights to the plywood ceiling. It should be about 1 1/2" shorter than your floor, to leave room for the vent supports.
The thinner plywood square at the base (in the photo) is the ceiling.
The short ~3/4" x ~3/4" pieces arranged in a six-sided pattern on the ceiling are the vent cover supports. I chose hardwood for these so they would be less likely to split. To figure out where they went, I put the cover in place and traced around it. The supports go about 1/8" inside this line, to allow for the thickness of the vent panels.
The 1" x 3/4" crosspiece that's just visible on top of the top brace (in the photo) between the two uprights is there simply to hold a screw on which the sensor will be hung.
I used Titebond II wood glue and finishing nails to hold all this together. The nails hold the pieces in place while the glue sets, so no clamps are needed, and they give a little added strength afterwards. Tip: When nailing the small pieces of wood (as opposed to the plywood), I recommend dulling the points of the nails. A light tap or two with the hammer on the nail points works well. This makes the wood less likely to split.
If your remote sensor is equipped for hanging, now is the time to put the screw in the crosspiece.
Step 5: Build Roof
I used a 12" x 12" piece of corrugated roofing for this because that's what I had. It isn't ideal, however--a flat piece would have been better, and something like a square baking pan (only with angled rather than straight sides) would have been better yet.
That having been said, the purpose of the roof is simply to deflect sunlight and rain. A shiny or white surface is best. And you want a space between the roof and the ceiling to keep any heat absorbed by the roof from transferring directly to the inside of the shelter. Short lengths of 3/4" PVC pipe served me as spacers between roof and ceiling, with 2" bolts to hold the roof together.
If you are using corrugated roofing and you have a choice, put the holes for the bolts at the peak of the corrugations, not in the valleys as I did, and just use longer bolts and spacers to make up the difference. You'll get less leakage. There were already holes in my piece of roofing, and the underside was painted dark, so I used a little silicone sealant around my bolt heads and hoped for the best.
Tip: A hammer and a stout nail are quite effective at starting your bolt holes in the roofing, though you'll need a drill after that to finish them.
Step 6: Mount Vent Cover
This is pretty easy. You will need a few more screw holes in the top of your vent cover, one in the center of each panel. As the aluminum is very thin, a hammer and a box nail will punch right through it. To avoid splitting the vent support pieces, pre-drill the screw holes in the wood.
Use somewhat longer screws to secure the bottom of the vent to the shelter floor, since you are screwing into end grain, which doesn't hold well.
Step 7: Install Remote Thermometer
Put your post as far from buildings, trees, and pavement as you can manage (see Step 2, above). Make your hole 2 1/2' deep, so the sensor will be sitting about 6' above the ground.
Slip the assembled shelter over the post. I used deck screws, four to a side, to anchor it. Nails would have done the job, but this way I can take it off the post if I need to.
Follow the particular directions for your thermometer to set it up. Mine needed the batteries to be put in both units while they were close together before the remote could be put outside.
This done, put the remote sensor in place. Reach up inside from the bottom and hang or set the sensor in the shelter. Then go in and shut the door, and read the outside temperature at your ease.
Note: If your thermometer housing is set up above bare, reflective ground (as mine is), you may find you get more accurate readings on sunny days if you install a floor as well. Tape sturdy foil (like a potato chip bag) in place and keep an eye on your readings to determine if it's worth doing.
I used a couple of pieces of aluminum lawn edging because I happened to have some handy. You want lightweight, reflective or white-painted metal or thin plywood for this. I made templates with a couple of pieces of old cardboard, used tin snips to cut the metal and screwed the floor in place. You'll need one of the sides to be easily removable so you can get to the thermometer unit to replace the batteries.