This project is still lying around in my workshop. The biggest problem I had with it, what kind of glass/plexy/etc. should I use on the top of the LEDs to be able to allow the UV light to go through it. Even I scored a UV integrator on the eBay (for ~$20) to be able to measure the performance of the LEDs through the chosen material.
On a day just pop into my mind that I was thinking in the wrong way all the time.
What if I turn the whole thing upside down?
1. I don't need any material between the LEDs and the mask.
2. When I put the mask on to the work piece I can see the alignment, not just guess it.
Now the next step is designing and building a new enclosure. Even I went further with the thinking. I put the whole thing into a flat closed box (without an lid what can be opened) and add a drawer for the work piece. This way it occupies less space, because I can put things on the top of it.