There are already plenty of great tool holders on thingiverse (I use several) for the Ender 3. I made this as a left-overs box to stick a few odds and ends in that I didn't have any other place for and seldom use or need but wanted to keep with the printer (spare cutters / bed clips / drive belt etc).
There are two versions.. one is 110mm long which should fit level with the end of the power supply and a slightly longer one that overhangs (no more than the rear of the bed / frame) but some people might not like the aesthetic of the overhang so I made both.
Mine was printed at 0.3mm layer height with 10% infil (0.6mm nozzle) in PLA... as long as you don't expect it to hold concrete or stand on it there is no reason to use more plastic than needed, There are already 2 holes spaced to allow you to use T nuts and bolts to mount it to the rear of the frame (Don't overtighten the nuts). I stood mine off the power supply by a few mm to avoid any rattles. Your mileage may vary. If you decide to try it out let me know how it goes.