We have a couple of the cheap plastic sledges which are pretty fast, but not at all steerable and break easily. I wanted a sledge which was strong, steerable and fast. I chose the waste pipe to give a minimal contact area on hard snow but a larger area as the snow deepens. The tube is also slightly flexible to allow bending for the steering.
UPDATE - This sledge works best in hard-packed snow. To make it work in deeper snow, follow BugsyandSpike's suggestion below and put a 2" x 1" piece of wood (with accordingly longer bolts) between the platform and the tube to give it better ground clearance. (There's a few other worthwhile mods in that comment too.)
It has been mentioned that PVC gets brittle and can splinter in the cold. I've given my sledge some pretty rough treatment in a couple of degrees below with no ill effects, but it's one to be aware of.
Step 1: Parts and Tools Required
The tools and other materials you will need are a drill, a saw, a large screwdriver, a wooden or metal rod and some duct tape.
I've used metric units here, but if you're using Imperial measurements, the cut size of the ply is 34" by 20.5" x 3/8" thick". The 40mm pipe diameter is just over 1.5", but I think you use 2" in the U.S. As a rule of thumb, 25mm = 1", 1' = 305mm, 1 metre = 39". There are 10mm in a 1cm, and 100cm in a 1metre.
Step 2: Preparing the Parts
Cut off 2 sections of tube, each 55mm long, and one piece of 42cm. Cut what's left in half; it will be about 125cm. Cut the board down to 86cm long, 52cm wide, and mark and drill 4 holes at 20cm intervals, 25mm from the edge. Line the board up with the pipe and drill down into it.
Use the adhesive to glue the short sections of tube and the end-caps into the tee as shown. Leave to dry. Don't glue in the longer cross-piece or the long side pieces yet. The end-caps stick out the side of the sledge as footpegs, and the pressure on these gives the steering.
Step 3: Assembling the Base - a Test of Skill and Judgement
Put a nut on this, feed it down the tube and screw in the bolt from the other side - the square nuts will catch against the curve of the tube and grip, but don't tighten fully until you've got them all in. Also, it helps to mark the dowel at the depth of each bolt-hole.
Start with the close ones, and work down. It isn't as hard as it sounds and I was able to do the last few in under a minute each.
Step 4: Adding the Front
Sand off any rough edges because the last thing you want if you wipe-out is a hand full of splinters A coat of paint or varnish wouldn't go amiss, either.
Step 5: Testing and Modification
The modification was to cut some of the base from the front, leaving more unsupported tube. This had the effect of making the front more flexible both sideways and up/down, making for better steering (although still not perfect) and a faster ride over bumps. It worked best on hard-packed or frozen snow, but got bogged down in soft snow because of the small ground-clearance.
By the time I got to test this, the snow was on the way out so it wasn't really able to perform to its full potential, but I'm pretty confident that this sledge will perform at least as well as the moulded plastic ones, with better stability and some degree of steering.
All I need now is some more snow so I can have another couple of productive days "working from home".