If you keep the speed of the dremel low, it shouldn't melt the plastic. Speed kills.

 

On your plastic, cover the area that you are going to cut with a layer of low tack painters masking tape and rub it down. On the tape, draw the outline of the hole that you want to make. Working from a pilot hole inside the outline, work your way around the line you made staying small and test fitting often as you near the pattern perimeter.

I like either the blue or purple 'Scotch' brand 2" tape.

 

When I took my fender off, I used a putty knife and heated it with a torch, it sliced through smoothly, and makes a nice straight line. Plus it doesn't spray plastic bits everywhere like a dremmel.

 

+1 on blue tape & hole saw or forstner bit depending on holes size and for oblong openings connect 2 hole saw/forstner bit holes with a sharp construction knife with a metal straight edge or free-hand but making multiple passes. Hot plunge cutting with a construction knife maintained at the proper temperature with a butane torch works for square/rectangular/sharp edged openings, and trimming off bits, for example, removing the rock guards from the stacion tubes (slicing on the rear flash line). Edges can be polished with a file and then aluminum oxide impregnated rubber tips than thread onto a mandrel for a rotary motor like a Dremel. The tips come in bullet, wheel, cone and cylinder shapes in different grits. Low speed is required for any drilling, cutting and polishing.

 

Hah! No such thing as slow speed on a Dremel, my friend. I have a variable speed and I think the lowest speed is like 8000rpm.

 

Thanks all for the recommendations. I'm going to give this idea a little more thought before cutting, maybe look at some other rear signal options. If I do go this route, I'll post pics and a description. Today, however, is beautiful and I just want to ride (and save the mods for rainy days). Thanks again!