I was looking for a good build plate holder I could screw onto the side of the enclosure my printer is in. I didn't find any so I remixed the best free standing one I found to add a back plate with some countersunk screw holes so that it can be cleanly mounted. The one I preferred was designed to be sat on a stand at an angle but It works flat too so I got rid of the notches, added the back plate and some holes and here it is.
I bought some PEO sheets so I needed more plate holder space. So added a 5 slot version. To fit on the build plate I had to make it a tiny bit shorter.
Update 11/26/23:
Replaced 3 and 5 plate models with the v2 version that has a reduced size back plate to reduce filament cost.