Traditionally Honoki was used. Reasonably close western counterparts would be alder or poplar.
However, if you are just making a shirasaya and planning on using it, you should probably not bother with traditional form. Shirasaya were not intended to be used as weapons, and in fact the rice paste used to hold the saya and tsuka together was designed so one could "break" them open again for cleaning. The rayskin and fittings and ito actually served purposes beyond just being pretty and providing a better grip. Shirasaya were just for storage.
If you want to make a "use" shirasaya, which I don't really recommend in the first place, you may want to use a harder wood such as hard maple with a very solid strong adhesive (wood glue or epoxy or whatever), make the channels a little wide and bed the tang with epoxy or acraglas or something. You might also want to make a seppa to go between the habaki and the tsuka, made flush to the edges so it doesn't look too tacky when the sword is in the saya.
Sorry if that wasn't the answer you were looking for, but hope I could help in some way.