Hi guys,
I don't know the best for your project but I can explain what I did for the OLRE16. First is the MASK, it's black PMMA.
Both sides are milled.
I let some space between the leds on the pcb to keep some matters between the leds housing. Led size is 1.5x2mm
On the other side(front side) there's some stripes which will fit inside the translucent PMMA, they will block the light between the leds, between the rings and between the rings and the oleds. .
Note: the olre16 top pcb(ring) has no component on the top except the leds and the oleds.
In blue are the back leds housing.
In Red there are the holes.
In Yellow, some stripes to block the light on the front, those stripes will fit inside the back of the translucent PMMA. The second part is the 'WINDOW', in translucent white PMMA, it's a LED special one, the same I used for the beat led window of the Seqv4+.
The back part will fit inside the MASK, in other word the base of the WINDOW will receive the MASK's stripes, of course the WINDOW's pipes are in front of the MASK's holes.
Then the Aluminum front panel comes to finish blocking the light and the pipes of the WINDOW will fit inside the panel, flush the surface.
When they are coupled
When coupled , assembled the thickness without the pipes height(front panel thickness) doesn't exceed 2.5mm
Voilà! I don't know if it will help you and It's surely an "over-engineered" thing but this is the only solution I found, and it works.
Best regards
Bruno