[particlewave]

I made horizontal passes, leaving strips of amber in place so that the routers guide had something to ride on.

Slow is the key. I set the Dremel to its slowest speed and made slow passes, advancing 2-3mm and retreating 1-2mm in in a push-pull motion in order to let the debris clear out. Too fast and the polycarbonate will melt instead of shaving away, clogging up your bit. Melting will also cause a divot.

2 tips;
1) use a thin lubricant like WD-40
2) use a sharpie to scribe guide lines (much easier to see)