Just treat them as if you were building enclosures for free standing speakers. Well, volume-wise anyhow.
Trying to build enclosures for in-wall creates some problems. It'll need to be very tall and very narrow which creates resonance problems. So you'll want to fill the top & bottom with high density fiberglass. Figure your wall is typical sort, then you'll have exactly 4" of depth to work with if you plan on making it flush with the typical 1/2" drywall. If using 3/4" MDF, then that leaves you with 2.5" interior depth. Not much at all!
Use something like boxnotes to figure out how much interior volume you need and what port length and the exterior dimensions.
I've attached pics of the in-wall build I'm doing pretty soon. It's Jay's in-wall crossover adaption of the Modula MT with Seas tweeter.