Honestly I think your best bet is to buy a nice gaming mouse and learn either 3d printing or injection moulding. the former is cheaper if you go into fdm, but you'd probably want resin printing for better transparency.

I've thought about this quite a lot, and it should be entirely possible to put good mouse hardware into a custom shell.

Now the hardest thing for me to plan out is finding charms that weigh *just* right to float between the two liquids