You can build a well sized solar air box that will help heat your house for around a hundred dollars or so depending on the materials you have at hand. It's not complicated but you will need basic construction skills and some time to work through the job.

Step 1: Size the Box

The first thing to do is figure out where you want to place the box or boxes you build. I have found out that bigger is better when it comes to solar air heater boxes.

Find the best and biggest not shaded south facing area you can.

Prepare whatever foundation and side installation mounts needed. In the photo I used 4"x4" posts to hold the two 3'x12' boxes and attached them to the side of the house using wood screws into both the window frame and strip glued to the side of the house. This has worked in high winds, rain, and snow.

The sun provides about 1,000 Watts of energy per square meter of space. That is about 10.8 square feet. For comparison, a sheet of
4'x8' plywood is 32 square feet or about 3,000 Watts capability! Notice that the space heaters you get at like Home Depot or Walmart
pull about 1,500 Watts each out of the wall.

So a sheet of plywood sized solar box can give you 2 space heaters of energy in theory. However, your solar box is probably not as efficient as a space heater, so the energy - heat you get out is actually less.

Parts you need:

Plywood or other backing...I used old doors.
Screws, nails and good waterproof carpenters glue
Guerrilla Glue - optional but it rules!
100' of 3 inch drain hose - about $55 bucks of you can buy 10 foot sticks at about $6/stick
Expanding foam - minimal expanding this stuff is the best foaming/fixing drain hose and filling big cracks.
Expanding foam - maximum expanding for the window openings
Tape measure - 25' for big boxes
14 mil woven poly plastic from ebay or your choice of covering. I used the poly and that is what I describe here.
Staples (3/8") and staple gun. You can use other fasteners like maybe roofing nails or something, but it is hard to beat a staple gun.
Plastic rope or cord to fix (staple) the hose to the back of the box.
Caulking for sealing up the window inlets
A jig saw or coping saw

Step 2: Lay Out the Box

The box layout is simple. Before going any further, determine where the cold air is gonna come in and the hot ais is gonna exit. Go to the spot where you are going to mount the box and actually measure and mark on the box exactly where the hose is going to exit the box.

You are just going to nail or screw and glue 1"x4" or 2"x4"sides to a sheet of plywood or what ever you use for the back of the box. Once you have attached the sides to the back of the box, cut out a slot large enough to accommodate the 3 inch drain hose exiting the box to the house.

The brown/orange outline shows the sides. The gray thick line is the drain pipe. On the drawing shown the inlet and outlet are in line, but actually they were offset as shown in step 2.

Step 3: Install the Hose

Next you want to uncoil the 100' long 3 inch drain hose. Before cutting it, measure how long it needs to be. Make sure to include the bends in the pipe, as well as the amount needed to get from the top of the box through your window opening.

Thread the bottom of the hose into the opening at the bottom. It will help hold it in place.

Wrestle the hose into the box, it is easier if somebody helps hold it in place. Take a 6-8" or so length of plastic rope and staple it over the pipe in spots as needed to hold the pipe in place.

Finally run the hose through the top opening.

Step 4: Make the Window Openings

You need to make an opening for your window to get the 3 inch hose into your house. If you want or need to insulate the opening, use 2 sheets of plywood and put insulation between them. I used 2 1/4 inch sheets of plywood and cut a hole in the using a jig saw.

Notice that the opening hole size for 3 inch hose is more like 3 and 3/8 inches.

If you don't need any insulation just use one board and cut an opening in it.

Space the two boards for the opening an inch or two. Tape up the sides well. Fill the void with foam. I used maximum expand
foam for this step.

Step 5: Staple on the Plastic

Before putting the plastic covers, make sure all the cracks in the box are sealed or you will lose heat. You can use glue, caulk or what ever you determine will work, but seal the cracks with a sealant that can take heat and weather.

After you have the box sealed, layout, measure, and cut your plastic to size. Be careful to leave enough plastic to do the job, to cover the box from edge to edge.

I used an Arrow T-50 staple gun with 3/8" staples. You can staple across the cords, the weave in the woven plastic to hold it on with minimal tearing in the wind. Alternately you can use some kind of furring strip and use that to hold the plastic down. Just make you have a good seal to keep the heat in and cold out.

Step 6: Hook Up the Box

Finally you can install the box at the spot you picked and prepared in step 1.

Install the box and run the hose through the window box in step 5.

Caulk the openings as needed.

You may need to seal the sides if there are small leaks. Duct tape works.

Hook up your computer fan, and start heating!

See more at my site: http://www.evsroll.com/DIY_Solar_Air_Heater.html