nice job on the glassing!! as for a small circular saw, I DEFINITELY recommend the Dremel UltraSaw. you can hold it easily in your hand. their blade is somewhat of a hybrid between an abrasive disc and a saw blade. it makes a lot of fine sawdust so be sure to wear a good mask. but the cut is very smooth.
the blade is forward of the motor, like the old-school Skil worm-drive saws, so you can see the blade relative to the cut line. you can rest the front of the saw’s “foot” on the shell, then very slowly pivot the saw down so you can plunge the blade into the shell exactly at the cut line.
good luck!!
ken