outdoor mattress cover for home wall

Hey all:  I have a twin sized mattress 6' x 3' x 1' which I have been using along with a bouldering pad for my back yard wall.  I'm looking for a suggestion for a waterproof cover.

A good solution will solve the following problems:

1. Keep it dry when it rains
2. Allow the air to flow in and out when I land on it
3. Prevent varmints from nesting in it
4. Avoid mildew and Mold.
5. Bonus: doesnt grab my foot when i fall awkwardly and need to stumble around.

I have had it encased in a few of these mattress bags but they always tear eventually.  

BYSURE 1-Pack Sealable Thick 6 mil Mattress Bags Heavy Duty - Tear & Puncture Resistant Bag with Three Extra Wide Adhesive Strips for Moving and Storage,Fits Full Size

Does anyone have a better solution that doesnt have me carrying the mf into the basement each night?

What's your budget? Is this a $20, $100, or $500 question? Some thoughts/ramblings:

• Could make a vinyl cover (like the material that's on gym mats in elementary school gyms) for it that covers the whole thing. 
• Keep doing what you're doing.
• Move the wall inside
• Cover the whole wall area w/ tarps or build a big covering structure
• Stand it up against the wall when you're done
• Vermin hate dryer sheets, especially the cheap smelly ones
• Keep it off the ground when not in use
• Uhaul sells twin mattress bags for $3. They're fairly thin.

My 2 cents: put it in a mattress bag with some dryer sheets. Wrap it in a tarp or make a vinyl cover, then store it up against the wall off the ground. There's no silver bullet here.

I don't know what your budget is, but if there's a marina near you, they'll probably have an upholstery shop that can make you a cover with a zipper out of marine vinyl.  I've also used Craigslist to find random auto upholstery dudes to sew seat covers and such.  They usually have marine vinyl in stock, although your mattress might come back looking like it belongs in the back of an El Camino that hops.

Ditch the mattress.  Buy crash pads.  You will not win against critters looking for a warm home. You can prevent water leaking in from above but its hard to impossible to prevent it wicking up from below.  Mold will form under the cover on the pad and will poof out every time you fall.

Tyvek! - just get some grommets and cut out vent holes on the sides of the mattresses

Should be able to just wrap the mattress like a present with the stuff or get fancy and cut out pieces and tape it together as best you can to get it watertight.