I have cut holes like you are describing a few times. Agree that you need to use a fine tooth blade and move slowly to avoid cracking acrylic, but 1/4" isn't that thin, ime for this operation.
Also agree that you will have plastic everywhere, so if you can drain the sump, cut and then wet/dry vac the area you may get 99% or more of the plastic shavings.
I sometimes uses a wood hole saw to cut acrylic, a 1" bit would make nice rounded corners. Make a template to guide the cuts, points for the corner drilling and lines to guide the edges. I have used painters tape for this purpose, you can write on it or use the edge as a guide. I have a small hand held skill saw that I use for the straight cuts,, cut as close to corners as I can and finish the edges with a hand saw if I can't access the corners. I have a plastic hand saw.
If you put a plastic bag in the sump, tape the edges to the inside walls during the cutting can also catch a lot of chips, but they will fly into the next box, the stand, etc so if you can make a plastic bag curtain to shield those areas you might keep the dirty area a lot smaller than an uncurtained sump.
And Sand all the edges to remove them or else you will be cutting yourself when you work in the sump. The sanded edges also prevent breakage down the road. Go slow and steady while cutting, to fast will go rough while too slow will start to melt the plastic. Both wrong speeds can cause the motor to stall on my tiny battery tools.
It's easy to do well, good luck and don't stress and you will be glad once the clean up is finished.
