Finally I've tried it. I didn't transform my holga at all as I still want to use it with film. I just stick my aluminium plate inside the camera body with an electrical tape, along two edges, to prevent it from moving. Working with such a small plate is challenging, but it's a good fun. Wondering if it will be feasible with 35mm?
Exposure times not that bad:
Holga 120 GN
Optical lens 60mm
6x6 tintype, 40s exposure:
6x6 tintype,1min 15s (low light conditions, a bit overdeveloped) :