Update (June 2019): the plastic ND filters I bought are garbage. The image is all grey and there are ugly reflections from the sunlight. I recommend the TBS Jello Guard filters now. They're made of glass and work a hundred times better. Here's a modification of this holder suited for the TBS filters: https://www.thingiverse.com/thing:3663318
This is a Yi action filter holder.
It's designed to be printed from flexible filament and it takes 27x27mm square filters. Put the filter in place by stretching the holder before mounting on the lens.
This is not a standard size by any means, you need to cut the filter yourself.
I made this to try how my Yi would work with an ND filter. I used cheap 1.6mm cokin filters which I cut to size with a dremel tool. Unfortunately it's too dark during autumn, so I'll have to wait until summer to find out how much sense it makes to use it.
Although this is not strictly a remix, the shape is heavily based on Xiaomi Yi lens protector by A_Lappi.
any TPU (98A)
Print without support or retraction. I recommend using brim to avoid warping.