I don't know of any handheld devices that take C-map. I think your best bet would be to engineer a base for a chartplotter mount that clamps in place without drilling any holes. Drill the holes in the removable base, rather than the boat.
I've used HDPE (1/2" thick, white, solid plastic) cutting boards to make things like this. It's a great material to work with; it's readily available at grocery stores, completely weather proof, machines easily with woodworking or metal working tools, is cheap, and looks nice.
If you want some help with the design and fabrication, lemme know.
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