I used 2" pipe rather than 3" pipe because that's what it looked like the professional models use. I don't know if 3" would be easier to ride. I've only given it a few tests, but it all seems to work. I will say that riding this thing is not like riding a bike in that it takes a lot more coordination and balance! I think that's a good thing, but it will take some practice to be able to use it like they do in the online videos. I do like the way it feels more than the stand trainer -- more realistic and requires some real concentration.
I've added a video of it in action. You can see that it takes balance and concentration -- i've only done it a few times but am getting better quickly. Ignore the grunting in the background, that wasn't me, someone was doing P90X.
Step 1: Make the Rollers
I used a 7/8" spade bit to make sure the inside of the clean-out cap was large enough to accommodate the bearing (22mm outside diameter). This also helped center the bit in the end cap since the bit barely fit in the square opening of the caps that I bought. I then used a 5/8" bit to drill out the end cap to allow the bolt and hex hut to fit in.
Drop the bearing into the inside square space of the end cap, then epoxy it into place by filling the space around the bearing and the walls of the cap in the corners. Make sure not to get epoxy on the bearing or where the nut or bolt head (which have to spin freely) will be. you can scrape the epoxy off if it goes where it shouldn't when it is dried but it's easier if you don't have to.)
Let the epoxy cure for 24 hours to make sure it sets. Then insert the bolt from the inside and screw on the nut from the outside and tighten. I had to jam a screwdriver into the gap between the nut and plastic cap so that I could hold brace it while I tightened the bolt with a socket wrench. You should use thread lock, otherwise the nuts on the right side of the trainer will eventually unscrew themselves from the torque.
Once the bolt is in place, you can screw this assembly into the threaded adaptor and then glue that to the tube, one on each end. You need to either glue the threaded plastic cap into the female adaptor, or do like I did and put in a set screw (see picture). Otherwise the caps on the right side of the trainer will unscrew themselves from the torque when it is in use. I used a set screw because I wanted to be able to take the tubes apart in case there's a problem with the bearings at some point.
Step 2: Build the Frame
Cut the lumber, then drill holes for the three rollers. I clamped the 2 2x4s together and drilled through both at once so that the holes would line up. Drill the holes about 1" down from the top of the 2x4 so that the rollers clear the ground and stick up above the top of the frame a bit.
Step 3: Install the Belt
Getting the tension right is a little tricky. I actually had too much tension at first, and just widened the front axel hole to allow the axel to move back a 1/4 inch or so. It seems to work well so far.