i used to have a toddler mattress with a sheet over it and Gia loved it, however the dimensions and rectangle shape of the bed didnt support her many sleeping positions. 90% of the time she was hanging off of it in some weird way, so i finally opted for square or round beds. i was also disappointed in the sheet selections that i came across - and that was one of my main reasons for going this route.
I've tried a crib mattress in the past and although the size seemed to work ok the mattress itself didn't last very long (it compressed quite alot after a few months) for a 90 lb dog. I would think that a better quality mattress would likely last longer, but for the cost, I've discovered that these 2 things seem to work the best: 1) a good quality custom cut foam from a foam/mattress place 2) queen sized feather bed folded into quarters. Both of these fit quite nicely into most XL dog bed covers, they seem to be lasting forever, and the dogs select these beds over any other others in the house
Kramer has one that he uses at night and does very well with it. It is waterproof, has sheets that fit it easily, and since he's a smaller dog now (50# range) it fits him well.
I thought it was a really good deal, too. He also has an air bed for daytime-there is a thread in this section under Greg Jeffers website or something like that. VERY nice for a senior.
I let the other dogs share the other crib mattress I got at the same time (Walmart) and two of them will lay on it together, or one alone. Sometimes parts will hang off, but they seem comfy that way. And sometimes they will roll off and fall to the floor and then casually toss their paw on the bed, like I meant to do that.