Summary

original File:
https://www.thingiverse.com/thing:1171411

this thing is hollowed out and ready to insert the innerds of a Cassio F-91W
the Buttons can be rewired to tactile switches (4.5x4.5x3mm)
the screws of the sacrificial watch can be used to close the new case.

i'd recommend to use the screen lense file as a template to cut out something from thin clear acryllic.