I've used CVS bed mats that are pretty similar to the Goodnites/Drynites, but without adhesives, and possibly a bit bigger. In my experience, they are good for catching press-out leaks like when you want to sit or lay down in your soaked diaper, but can be unreliable for wetting while laying down, especially with a leak-prone diaper.
I have had times where, wetting while laying down, it leaks all out the back of the diaper, soaks the pad up my back, and the pee continues to creep or leak up my back off the mat and onto the bedsheets, leaving a massive wet spot. This was using drug store medical diapers though, so they were already prone to leaking, especially when wet while laying down. I have had better luck wetting ABDL diapers while laying down, haven't leaked as much (coupled with being familiar with the capacity of the diaper, knowing how far is too far to push).
I now use reusable bed mats and had a similar overflow problem when I used smaller mats, heavy leaks would follow my back up and over the mat, wetting my bed. I have since bought longer bed mats that can reach my shoulders/neck, which I am more confident in. (If you use a smaller mat but only for your upper body, you leave yourself vulnerable to leaks through the leg guards)
And a waterproof mattress protector is an excellent idea/backup. It gives a little more peace of mind even if you leak and even if some gets off the mat. But I once found it let my pee seep through to the mattress when I had a catastrophic leak that soaked my bed mat, creeped up my back, soaked my sheets, soaked the protector, and left a wet spot on the mattress. That was not fun, and one of the reasons I bought the longer bed mats. So only rely on the protector for small leaks, not puddles.
