Unfortunately, I seem to be missing some of the photos of the early stages of the project. If I find them, I will edit this post and add them.
To keep the pipes upright, I cut 3/4 inch EMT electrical conduit into 12 inch sections. I planned to use a forstner bit to drill holes through the plywood in three rows of four. The EMT would be fitted into these holes and stick out from the plywood to create arms that would keep all the pipes from falling over, and keep them separated into bays so I could keep everything organized.
forstner bits were either too large or too small to make a good fitting hole for the outer diameter of the EMT, so I went with the closest size that was on the small side, and cut a slit in the end of the EMT with an air grinder and a cut off wheel. By removing this little bit of material, I could pinch the end of the EMT so that it could fit into a smaller hold for a snug fit.
Once the glue was dry on the strips of 2-by, I started drilling the holes for the EMT. I didn't want the EMT poking out the back, so I set the depth of the holes to stop just shy (maybe 1/8 of an inch) of going all the way through. I used a piece of masking tape on the drill bit to help gauge when to stop.
I would need four EMT posts (plus the wall the rack would be butted up against) to make four bays for my pipe storage. I made the two outermost bays slightly smaller, and the last bay slightly larger.