What you could do is make the jig the same size as the full insert or the minimum amount to be cut.. Then just have some different thickness rings/spacers that drops into the jig before the insert, then the insert goes on top and will expose the amount to be cutoff
I'm making a nesting insert prototype now - only concern is durability with thinner walls, but I'll do some destructive testing to see if that concern is valid.
A reverse cut would make for a smaller guide, but unfortunately, some of the inserts out there don't come in a uniform width, so we'd need multiple spacers and measurement steps for a pair of foams.
I'll post up some pics of the next iteration tonight. Thanks for the ideas and support - sorry to keep spamming your thread PW!