Thank you Ed.
I started with an assortment of PVC sheet material to experiment with.
It was easy to mill a cavity in two pieces (shaped roughly the size of the sheath) for the blade and spring plate, but a router would work too.
The two halves were later joined with seven #4 SS screws, then the final shaping was done on a band saw and router table. Hand finished with wet sanding, using 2000 grit as the final grade, then plastic polish to spiff it up.
I was originally planning to just bond the halves with PVC cement, but wanted to be able to open up for improvements or modifications on this prototype.