sizing on korkers

[scoobe]

Hi guys, I'm looking to pick up a second pair of Korkers so that I have an emergency backup and/or a pair to lend to a friend (so many times I could not invite people with me because they didn't have proper footwear!). I will be using these with both a bootfoot and stocking foot type waders. My normal shoe size is around 9-9.5. My current pair of Korkers is a M but is a bit snug when slipped around the boots. It's a tight enough fit that they stick on the boot unstrapped and I have to pull them off with a lil bit of effort. I think I tried on the L once and it was too big. This may have been another brand product, though, don't remember for certain. Should I just go with the M size again or should I try the L? It seems like I'm right on the border in terms of sizing. I'm just afraid the constant slipping on and off will flex the rubber and eventually cause it to crack and fail. Any recommendations?