Various people have built zynthian rigs in photographic cases and the like. The issues are primarily about power supply. Batteries and UPS hhave been used but it’s an important aspect and that seems to set the overall agenda. The Pi itself doesnt really care as long as it get enough volts and current but it’s very easy to concentrate on other aspects forgetting this.
The other issue is the housing itself. If you build in a case to allow access to USB, ethernet, HDMI & Power supply you realistically have to build the Pi in a corner of an enclosure and carefully cut holes for these connectors or else you have to run cables and connectors which make things involved
Lastly there is access to the SSD slot. The early zynths require disassembly to allow you to change this which given a spare is a good idea, just in case, is a LOT of work in the middle of a set if you want to change it.
An honoury mention of straight to Amp zynth’s is probably in order. I use a couple of these as easy to install electric piano’s where a simple hifiberry amp card is used instead of an audio card. When linked with a battery pack I get about an hour and a half of 60W piano with no mains lead in sight.
Its so useful nobody ever notices it !
Have a look at Success Cases to get some ideas of the weird and wonderful solution people have produced.