This would probably get more answers in the cartridge
mod section, but I never had acceptable results with blue foam and 401's (it's great for 510's tho).
On my 401's I use the straw and scotchbrite pad (plug) method with great success.
But, if the 601/602 is bigger and you want to try blue foam, try using a straw that is 3/4's the length of the inner cart and insert that into the inner cart. Then stick a plug made out of blue foam or heavy duty scotchbrite pad material and see if that works for you.
I'd probably cut a small cube (1.5 x 1.5 x 1.5 times the width of the inner cart opening) and see if that works.