First version of this tool consists of two 3D-printed parts: the top part holds the heatsink and UV LED, and provides some distance to the mask which will be placed in the bottom.

The distance is needed to have the light spread evenly, as I'm using only a single relatively high-powered LED, as opposed to an array of LEDs. A single LED seems to be a much cheaper option than so-called "parallel UV light sources", which consists of multiple LEDs and additional optics.

The bottom part is a simple square frame with embedded magnets. The top part also has a pair of magnets, and the mask can be clamped between those two parts.

STL files of the design are attached in the project repository.