It would help to know what you're use of the case is to be. If you're testing components a test bench- that's one kind, if it;s to have complete access to build a custom liquid cooling system, to refinish, and/or for cable routing, that's another.
One company making fully modular cases is CaseLabs. Some can actually be sent as a flat pack- and assembled like IcKEA furniture. Most designs offer an infinite set of combinations of door's, windows, radiator mounting panels, open grilles, and panels with hot swap drive bay- really anything is possible. The complete disassembly possible on some designs allows the option to respray to any color and the parts are sent in primer. These are all Aluminum and designed for serious system building. Some designs are very large and have compartments, for example one side can have a pristine motherboard /GPU/s and the other side has the liquid cooling system, all the wiring, power supply. Other versions have upper and lower compartments An interesting example of which is the Bullet BH4 mATX Case for $190. The same design as mITX is $170 and full ATX for $230- which is the one I'd choose. This is an interesting, horizontal motherboard design with a lower compartment for the PSU, and comes in colors. Looks very airy and can mount some large fans. Although this design is not sold as a flat pack, the number of screws visible all over mean it's probably can be fully disassembled- give them a call.
On another level, if the main objective is total component access, most conventional cases can be substantially disassembled: remove the side access panel, the top, front cover, and the offside panel and it the system could be fully operational and still have access for whatever finishing. Many cases today will connect some panels with rivets, and in that case, carefully drill out the rivets- to be replaced later by screws and it can be as you say, just a frame on a bottom.