Summary

These brackets screw onto the tops and bottoms of your doors and then allow you to pivot and stack multiple doors for either painting or storage. Allows you to paint both sides easily.

You need 4 per door and these run anywhere from $25 to $35 for a set of 4 on ebay/amazon. Print your own for much less
40g filament per unit so 160g for a set of 4
Cost per set of 4 around USD$3 ($3 verse $25+)

You can print 2 at a time on an ender 3 or even slightly smaller sized printer.

Print with no supports and 20% infill. Recommend 4 walls for extra strength.

Use:
Screw to top and bottom of doors with a single 1 inch+ screw
Paint top of first one, pivot against wall, paint bottom - Done
Put second one on top and repeat

Similar concept to the below:
https://www.amazon.com/Saker-Painting-Interior-Exterior-Stacking/dp/B09HSHPSDL