I think she's having some problems with her site - it took me a while to be able to purchase the cover I wanted (storm @ twilight, yum!). But it's worth the trouble IMHO. Her covers are just gorgeous!
I think 1 layer would be fine for daytime. Not sure I'd try it for nighttime. But y'know, Lisa Marie, Lia is really really nice - if you explained Joe's knitted wool problems to her, she might make you a custom cover.
ETA: unless Joe is HUGE, a medium would probably fit him. FWIW, Ian weighs 23#, has a 20" waist & 18" rise, and is about 33" long...
They do run a bit big - Ian wears a large in most other covers, but the large that I have from Peenut Pants is huge on him. That's OK though, he'll grow into it.
Oh, and they're made with wool interlock, so they are somewhat stretchy.