hi Peter,
If you use a 2 part epoxy mix you can pour into a cheap, open topped pvc box.
They are in fact called 'potting cases', its usual, as they are so cheap,to leave the box as part of the potted assembly.
You could pour upto the level you want, let it harden, then trim off the excess pvc box with a utility knife.
It a 'once and for good' method.
There are silicon based potting compounds, which are semi-permanent, it can be cut away, the module repaired then resealed.
The best way I have found is to get a small IP65 box [water resistant seal].
Cut out a rectangular hole in the top cover, [ the size of your LED outline] cover this with 1-2mm thick perspex or polycarbonate, on the inside of the lid, seal and secure the perspex.
If you use a 'red' filter instead of the perspex its even better.
Regards
EricG
edit: to clarify, when using the water resistantIP65 box, no need to pot the pcb/assembly.