What I would do is use/make a template first instead of risking any screwup on a complete sump. The hole cutting template could be made of 1/4" acrylic/pvc/wood or whatever is not too expensive and easily machined. Take the template and stick it onto the sump where you want to make the hole with superduper tacky two sided tape. Then router out the new hole opening on the sump with a flush trim bit. If you can make the hole in the template you can make the hole in the sump without fear. It should turn out melt and chip free.
This is all great if you have a router.
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