go to a local sign shop or any "plastic" jobber.. you need the .250 Lexan (is actually .220 or so) and just get a chunk of scrap from them..
Use the outside ring to trace the new piece
Cut out using a jig saw, coping saw or even a power miter saw.. (what I had to use)
The miter saw you just got to start running straight cuts to the trace line.. from square to octogon etc till you get the points trimmed off to a basic round shape.. (100 tooth 12" is the blade I used)
remove and clean the cover of all old cover and sealant
put a nice seal of "ultra black" sealant on cover recess (do not use plain RTV or silicone sealer, it will degrade from oil.. you need the "ammonia" based sealer, no vinegar smell)
put the new "cover lens" into housing and weight it down over night (do not drill it yet)
after the sealant sets up, you can take your 1/8 drill bit (think that is the size, make sure) and drill the new lens from the back side (use existing holes in cover for guide) Take your time and you wont chip or crack the lexan
clock the outside ring to the right place and assemble the thing.. I spot of ultra black on the screws will stop any leaks on that part..
should be golden....
if you are unable to locate the Lexan, I do have a chunk I can send you...