All I can say is FaaaanTastic Job! Especially for your first Openscad ! Awesome. I haven't made a box yet, but will be using yours soon!
As you've thought about and mentioned already, the only "improvements" would be to add miscellaneous hole capabilities, like for screws, fans, access ports for USB, ethernet, power, etc. and possibly standoffs strategically positioned to mount 3d printer controller boards like the Azteeg X3, Rambo, Raspberry Pi. With this being your first Openscad Design I can't wait to see what you cook up next! Thank you for sharing your design!