It depends in part on how clever you are at wiring up components, I assembled one with parts from Slow Chinese marketplace aliexpress and faster delivering Amazon. Doing point to point soldering of a minimal all-in-one board.
The Zynthian basic starter kit simplifies the process of soldering together the special elements that interconnect things, you only have to solder some components onto a PCB board… Amazon is one fast delivery source of Codec boards, they were the cheapest source at the time, doe a couple of consumer grade Audio Injector boards. and LCDs I have used.
Pandemic era component lead times (up to a year for some electronics) seems to have effect availability of raspberry Pi’s. One local retail chain only has the most expensive Pi 4 on hand, with other Pis only available in ‘starter’ kits.
Major electronics suppliers, Digikey, Mouser, Newerk or Arrow have a different available Pis, as well as the chassis mount MIDI and Audio connectors.
There are plenty of DIY case examples in the Success Stories posts.