I carved my new filter out with an air tool with an abrasive cone, a little at a time until it fits. Then I put cling wrap (Saran wrap) over the carb area that you need to keep Proseal from sticking to. You need to tuck it into the corners around the accelerator pump protrusion so the Proseal will mold into the area nicely. Put the filter on with a gob of Proseal and secure it into its natural position. After it cures remove the filter and remove the cling wrap. Reassemble.
If I’m not mistaken the cutout is only the inside part of the filter, maybe a 1/3 or less of the thickness of the filter. The outer edge is untouched so after the plate is bolted on you should have a tight seal without needing any proseal, correct?
If you have the old filter use it as a template. It will take a bit of tweaking to get it in the right place. Once there I use a paint marker to mark the outside top of the filter and the fab to index it. I put it in two spots either side of the cut. After the Proseal cures, you have index marks that make it easy to re-home the filter. Nail polish would probably work just as well.