Under Desk Drawer 2.0 options:

• 4 drawer bottoms. Hex (Honeycomb), gridfinity, gridfinity+hex or solid bottom 
• 4 different handle types 
• 2 types of keepers, inside drawer or back of drawer mechanism 
• Optional divider insert

I really like the original Under Desk Drawer model, thanks @Paul_318082! I also like the drawer stopper idea from @Zakoplex.

I decided to improve on both designs with some of my own ideas:

• The under desk drawer has an optional solid bottom, preventing small stuff from falling out
• Stoppers (or keepers) are integrated in the rails to keep the drawer from falling out
• On the back of the drawer body there are two sliding locks. When these slides are closed the drawer won't fall out when opened completely. To remove the drawer you can simply open these little locks and take the drawer out.
• Because the stopping mechanism is located on the sides, it doesn't bump into stuff when opening or closing the drawer.
• Separate swappable handle (hex pattern & other)
• The rail includes an additional thin layer of plastic at the top. This allows for smoother sliding.
• The backside of the drawer body is made slightly lower than the rest. This prevents it from making noise (due too friction against desk) when opening & closing.
• An optional divider insert which allows you to divide the drawer in two halves 
• Gridfinity support

This make gives a good impression of what the printed model looks like:

https://www.printables.com/model/453163-under-desk-drawer-20/comments/805926

To attach the handle, simply insert some M3 bolts or small screws through the drawer body holes from the inside and screw the handle on.

A nice feature is that the handle can be printed in a different color, allowing some personalization.

I hope you guys enjoy it :) I'd love to see your makes, questions and comments!

Printing:

All parts can be printed without supports.

Update 20-04-2023:

• Added gridfinity drawer with hex bottom

Update 18-04-2023:

• Added stopper sliders which can be closed from the back of the drawer instead of from the inside

Update 16-04-2023:

• Added divider insert to divide the drawer in the middle
• Added drawer body with hex pattern bottom