Thanks for the info.
I did a quick page through of Carl's book before posting, but didn't study all the circuit descriptions.
I was hoping it would have a picture pointing to that square opening and say what it was for. Especially in the Race Prep section where this filter conversion could be mentioned.

I have my PG out for winter inspection, and was considering this filter conversion.
I think I will keep the PG screen filter (and deep pan extension). Since I will inspect pan every year.
I will keep all the dimensions I measured so I can make that adapter in the future, if I ever decide to do it.
This is my preliminary design, No guarantees !