Since I already build one sensor with hot glue form and it was half failure now I'm willing to do it better. Since TSL235R are hard to get here I need to ship them from US each time. I have few more, but I cannot let myself experiment with many ideas because I will quickly run out of them.
My first idea is using plasticine form (simple to use, easy to get and cheap) and then fill it with some kind of moldable polymer or resin. Then remove plasticine and cut parts that stand off. The advantage here is that plasticine will ensure me the parts will stay in right position (this was the problem with last sensor, part moved right after hot glue filling).
My other idea was to use two separate PCB, first one (A board) with TSL235R and seconds one (B board) with LEDs.
Then connect them under right angle.
This would require some kind of filling anyway or at least something to isolate external light.