For a Do It Yourself Project, here are some thoughts:
1) A rack that mounts to the bottom of a shelf (using otherwise unused space). Can build it from 2 pieces of 1/2" thick hardwood or standed 1 by (typically 3/4" to 7/8" thick).
Make it about 5" inside height to handle the height of most guns (increase if needed for your specific guns), using the same wood for the side pieces as the top and bottom.
Using a 1/8" thick saw blade (standard carbide circular saw) cut matching slots 1/4" deep into top and bottom (might consider cutting the top and bottom out of 1 piece of wood and cutting the slots before you cut them in half to assure they line up) Space slots 2" on center (you can go as narrow as 1/2" on center if you want to be able to adjust them for each gun, to 2" on center to cover most handguns (except larger revolvers)
Cut a set of 3/32" thick basswood or hobby project hardwood pieces to work as slots.
If you want to hold the barrel of a specific gun up, cut a 2 by (1 3/4" thick) piece of hard wood to make an underbarrel spacer
2) For hanging from the back fo the door, I've always liked hooks that are designed to hook into the trigger guard and allow the gun to hang upside down from the hook.
3) You could add a board with a series of belt-straps down the back of the door to store handguns in holsters as if they were on a belt. Depends on how you feel about storing firearms in holsters, but it does keep them with their holsters if you need them (like rotating your daily carry pistol)