try going to frys or some electronics store and get a cheapo soldering gun and take a hammer and flatten the tip, then use it like a hot knife and cut away
ding ding ding!
We have a winner!
A cheap 25W soldering iron will do instead of a gun, which is usually 100W or more.
Hammer the iron tip flat- and you have a hot knife, great for making large dia holes in plastics. If it is too hot and smokes instead of just neatly melting a clean cut in the plastic, you can always dial the temp of the soldering iron down by putting an incandescent light dimmer in series to the AC mains supply.
Rubbermaid containers are made from a rather pliable plastic that will cooperate with holesaws a lot better than the brittle plastic found in cheapo dollar-shop storage tubs. If you try to cut a cheapo tub with a holesaw, it will split or shatter.
I
HEART my Dremel.
A Dremel with a small drill bit, run at high RPM and drawn sideways along the cut line will also work to melt a cut in some of the more brittle plastics, but if the bit jumps in the work, it may split or crack. A sanding drum in the Dremel is great for cleaning up and smoothing out cuts made by any means.