I have been looking for a cover for the rear mattress that covers the engine hatch.   This one tends to get dirty when camping.
Carolina pet company makes one that is nearly a perfect fit.   You have to buy the mattress to get the cover but the cover is built for dogs and is rugged.   When I ordered it from http://www.carealotpets.com/ I was charged 59 dollars and shipping was free.   It is a tight fit but it does fit with some effort.   Gary
http://www.carealotpets.com/item-detail/?ItemID=000000000148923&UnitCde=1Four

http://www.carealotpets.com/
Season Jamison Dog Bed Blue 48 in. x 36 in.