Welcome to the forum...
We don't have a problem with a non-standard mattress anymore. But when we did we would use two flat sheets instead of using a fitted one on the bottom. Just make hospital corners and tuck it in all the way around. If you don't know how to make hospital corners, ask a nurse or any Vet, they can show you.
Russ & Kim
2014 Alpine 3600RS
2007 Dodge Ram 3500 5.9 4X4
2008 Harley Davidson Road King 105th Anniversary
(2011-5 bad year)(2012-40)(2013-45)(2014-23)