The rack couldn’t be much simpler to build. It consists of a pair of arms and a pair of legs made from 2x4s. Each arm and leg set gets bolted together, and then the arms get bolted to the stud wall. (Place washers under the bolt heads, between the boards, and beneath the nuts to allow the parts to move smoothly.)
You’ll also want to add a pair of keeper blocks that get screwed to wall studs to hold the rack in place. When you need to use the rack, the arms pivot out from the wall, and then the legs pivot down to bear against the garage sill plate.
The easiest way to make the rack is to cut all the parts slightly long using the Kreg Square Cut. Then you can clamp the pieces in their rough positions and measure and mark the final length. After cutting them to length, it’s a good idea to add a notch where the legs meet the sill plate. While you’re at it, round off both ends of the arms and the top ends of the legs to soften them. Both cuts are easy with a jigsaw.