If you have a GoPro camera, you may wonder if filters normally used with DSLR cameras could be used with it. The good news is that yes, you can, and if you have access to a 3D printer, a handy adapter can be printed for this purpose. This adapter was originally seen on Reddit, and you can get the print file here if you’d like to make one. It’s designed for 52mm filters, but the design should be able to be modified for whatever size you need.
I’ve actually done something similar to this 3D Printed GoPro Filter Adapter, instead cutting out of wood using a CNC router. You can see my results, as well as video of it in action here.
Alternatively, if you don’t have access to “exotic tools,” you can simply buy a GoPro filter kit with three lenses from Amazon for just under $12 (as of this writing). This is certainly the easiest option, although possibly not as fun!