The mute I made for my Walter Mahr 5-string viola. It has three holes instead of two so it can fit in the centre of a 5-string bridge. It was designed to fit my 16'' instrument, but you may find it fits a smaller 5-string viola or 5-string violin with some re-scaling in the slicer.

I printed it in TPU at 100% infill. It required a fair bit of finishing afterwards with a hot knife to refine the holes and the groove that clips over the bridge. As you can see from the photos, I never managed to get it perfectly smooth, but although it's ugly it does the job of muting very well. You'll probably want to experiment with your print settings and orientation to find a result that requires the least amount of work afterwards. I've never been very  good at printing TPU so please let me know if you find any good tips for making this model.

You'll need to cut three slices into the mute with a scalpel so that it can be fitted onto the strings, as per the diagram.