I use the 3M spray on Adhesive. Its a contact adhesive, so you spray it on both surfaces, let it dry till just a little tacky and then stick em together. They make a couple different versions.
Super 77 is a good general use one and holds well. Last time I got the Hi-strength 90 to glue some mini cell foam to my dry box and a couple of Rocket boxes I use for my captains boxes and it has held up awesome through a grand trip and a Lodore trip and living on my trailer in the elements for most of the winter. I actually carry the can in my tool box now just in case I need to stick some stuff together. Most hardware stores carry both the 77 and 90 stuff.
Not sure what your sleeping pad is made of, but beware that some foams might dissolve with the solvents in most adhesives. 3m makes a version for those foams that isn't as strong but won't dissolve the foam. Its most styrene based foams where it is a problem though, so you should be fine using standard adhesives.