Thanks Slotbubba, both for the info and the gracious offer. I'll give some EPDM a try, no need for you to go to any expense or trouble.
So far, I've been doing tires for 440x2's. As for sizes, I don't have a tire gauge yet, and haven't braved the weather to run out to the garage and grab my calipers or mic... So I'm just grinding them down smaller than stock tires, and matching pairs by eye. I know they're far from perfect, but as long as my cars aren't hopping out of the slot and are getting better grip than stock, I'm happy.
I've also been experimenting with sanding down regular silicone tires
, and recoating with flowable silicon. I haven't got to do any testing to see how they perform or how durable the new ''coat'' is, but they sure look pretty.