I think that would be great - I'd love to see one that allows for HATS to be installed stealthily - modify the design to lower the entire metal area over and between the HAT mounting holes to a lower flat level that still contacts the CPU and USB controller (with slots for camera and display ribbon cables plus the POE header). Given the Pi 4 has such low connectors you would probably not normally need a GPIO riser to fit one.
The plastic top would still be level, but could extend down three sides if the 'shelf' makes it too hard to cast. Status displays, TV inputs, UPS batteries would all love this option, and if you didn't need it you can always install a heat sink (or fan) to further help cooling as the top will still cover the void. Throw in a 3D model of the plastic bit (so people can print their own custom tops for taller hats) and you have a makers dream case ;)
If someone has a mill they could probably mock it up... anyone?