Skynetbeta

Find a mouse to use as a base and disassemble it. Use a Plastalina Modeling Clay to sculpt a custom surface for your hand. Take your sculpt and create a silicone mold then use the mold to cast a shell out of a urethane or epoxy. Attach the shell to your mouse and reassemble now you have a custom mouse and a mold should you need to recast in the future.

SLA printing water cooling components is possible . You need to make sure that your part is fully cured and that your resin is non reactive once cured. I would suggest a resin rated for food or medical use. In regards to FDM printing of water cooling components both material and post processing is key. I have have gotten the best results when printing in nylon carbon fiber. The key is to print with 3 mm thick walls and in an orientation to prevent shearing of the layers. After printing infiltrate your parts with a low viscosity epoxy resin and sand to your final dimensions. Alternatively if neither of the above methods is available to you simply print out of your material of choice then make a silicone mold. Once you have your mold you can cast out of a clear resin or urethane. Casting your parts also allows you to add thermal additives making the fabrication of heat-sinks possible.

I would suggest using a 3D print to create a silicone mold then casting your fixtures out of a temperature resistant epoxy. ex https://www.smooth-on.com/products/epoxacast-670/