how heavy is it? must be HUGE!

i'd go for silnylon as the material itself is waterproof and can be had cheaply enough. any decent sewer could sew a cover for you.

Is memory foam open cell? If so, you will want waterproof material, but you may find it difficult to roll up as tight as the fabric will also hold in a lot of air. That is why Thermarests have a valve.

You could get heat sealable fabric and do it yourself with an iron if you don't know how to sew: http://www.seattlefabrics.com/nylons.ht ... %20Ripstop

Or get some Tyvek and just use glue or Tyvek tape.

Or buy a window insulation kit and use the plastic and tape that comes with it.

But man that's going to be heavy.

Yeah I'm pretty sure it is open cell. My bed mattress is 12 in. of memory foam, and if any liquid gets on it, its pretty much ruined. Although I do get the best nights sleep ever, every night!

When I was young and poor, say 1968-1974, I always made pads out of 1" to 2" open-cell polyurethane furniture foam that I bought at the local sewing shop. If you put a covering on it, even a thin groundsheet, the moisture will end up in your sleeping bag instead of your pad, but some prefer that. When I was a kid, we'd cut off all foam that wasn't directly below our shoulders, back and butt.

If you dig hip and shoulder pits (I often do that AFTER my 1-person tent/bivy bag is set up, by reaching underneath the floor as I am lying in my bag), or use extra clothes under your legs, small of back, etc to take weight off your contact points, you can get by with a much thinner pad. I have been using the same full-length ridgerest on back-pack trips for 16 years. It takes an extremely thick pad to make up for a poor sleeping spot.