The hardware just arrived. For lack of experience building mirror cells I ordered two different sets of compression springs to feel them out. One set is substantially stiffer than the other, but both are fully capable of lifting the combined weight of the upper plate plus mirror at as little as 0.25" of deflection.

Pictured from left to right (top row first):

1/4-28 stainless knurled thumbscrews, 1.75" shank (collimation bolts)

Stainless compression springs, 1.5" long, 0.375" OD, 17.68 lb/inch

Stainless compression springs, 1.25" long, 0.6" OD, 37.49 lb/inch

1/4-20 stainless knurled thumbscrews, 1.5" shank, (locking bolts)

8-32 stainless socket-head machine screws, 0.75" shank, (mounting bolts)