You could probably lift the display a bit with spacers under the factory hinges (inside the bottom chassis), maybe adding a cable extender if the factory cable is too short.
Swap the top input cover with a thicker one (corresponding to the hinge spacers in thickness) that has the mechanical keyboard and keep the entire bottom chassis unchanged. Use the opportunity to improve cooling or sound with the thick input cover.
The bottom chassis is going to be changed because its to flimsy in my opinion.
Then im going to make it wider as well so a gasket or seal of some kind can close off the usb ports
Being that heat rises, would it not be better to put the fans above the mainboard, instead of below it?
So, mainboard at the bottom, more space for fans to cool the CPU, RAM and Battery.
Then some water proofing and then the mid-plate and keyboard.
With the extra space, one could also put in better speakers.
Are you planning to also make the top cover deeper? With it deeper, one could then have space to replace the FW display, with any display one wished. The deeper bit needed for a circuit board that would adapt any display to the FW display pinout.
With the bottom deeper, one could add connections to all the debug ports. I.e. Able to flash coreboot firmwares, change USB PD firmware, have console port access to BIOS etc.
Essentially making the mainboard un-brickable.