For building my UV sensor I bought a circular piece of quartz glass (it's used as window in some high temperature furnace for baking clay or so - don't really know how those "ovens"
are called ), but I know, the quartz glass don't block UV and IR very much so it's ideal for sensor that measure UV .
I've got 2 ideas for now, picture tell more than I could write
I'm more for v1 because if rain, the water will flow from the glass, and leave a little of residues, unlike in v2 when rain water vapor there will be much more leftovers, and I would have to wipe it more frequently...
As for silicone, I don't need to mention to use UV-resistant one
Acrylic - you mean PMMA/plexiglass, right?
PMMA is not very good for UV, it actually block it (below 300nm according to wiki), that's why in those big sandwich toasters
where you can get tan, the fluorescent tubes and glass panels are made from quartz glass.