Are the gel tray and the tray support portion of the gel box UV transparent? If yes, what is the approximate % transmission at 300-320 nm?
No, the gel tray and tray support section of the gel box are not UV transparent.
We have in the past made gel trays and the corresponding gel box support from Solacryl which is a "high transmisson" UV transparent material. Solacryl worked really well for viewing gels on our UV-B transilluminator so I would recommend it if you were looking for UV transmissive acrylic. I don't have a datasheet for the material though so I don't have a % transmission at 300-320nm. You could check with the company - Spartech - for a datasheet with % transmission.