This is a pretty simple structure so I won’t go into a lot of detail. I used 2×6 lumber because I happened to have several short pieces on hand that were just the right size, but 2x4s should work fine too and would be a little lighter. My side pieces are 34 inches long and the ends are 19 inches long which leaves a gap of 16 inches in the middle. Note that one end needs to be ripped down 2.5 inches lower than the other because the drum needs to overrun the box slightly so that the coarse materials spill out of the drum. Since I used 2x6s (1.5 x 5.5) my spillway end piece was 3 inches wide. If you are using 2x4s (1.5×3.5) Then your end piece would need to be one inch wide.
Note that these dimensions assume that you are using the same diameter rims that I used. If you use different size rims then obviously you will need to modify the dimensions to accommodate your particular arc.
To align your rollers place the finished drum onto the roller box and then mark your roller positions. You may want to attach the rollers first with simple drywall screws (as I did) to test their positioning and then drill and bolt them later once you are happy with the alignment.
Once the roller box is complete you are ready to try the trommel out. Mine fits perfectly on top of my wheelbarrow which is handy so you may want to take your wheelbarrow size into account when planning your dimensions. You could also place the trommel on sawhorses or make permanent legs if you prefer. Just make sure the output side of the trommel is slightly downhill from the input side.
If you have a large quantity of material to screen you may want to find a buddy who will keep turning the drum while you continue shoveling material.
Once I tried the trommel out I discovered that if you pile a couple of shovelfuls of material in the uphill end of the drum and then turn it yourself some of the coarse material tends to spill uphill and out of the drum. To solve this problem I made a simple partition out of plywood to prevent this from happening. I don’t think this would be a problem when operating the trommel with a buddy since the material wouldn’t build up into such a large pile as you have when working alone.
This is indeed very simple equipment that would help an average gardener make compost. You could copy this project as it is or you can use it as inspiration. Be creative, look around you. What do you have in your garage or storage you think you could use to make compost? How would you build your own compost sifter?