BUT-H1 facemask filter frame

This is a frame for a replaceable filter to fit in the [BUT-H1 facemask](https://www.vutbr.cz/en/but/news-f19528/covid-19-protection-halfmask-but-h1-d197642) made by the Brno University of Technology in the Czech Republic.

The frame consists of two parts which form a zigzagging grill. The filtration medium is weaved through the grill to increase surface area, much like in commercial vacuum-cleaner HEPA filters and similar. The whole filter can be sterilised by boiling water if printed from ABS (and if the filtration medium itself can be sterilised this way).