If it can be done, it has been/will be. Humping the bed reminds me of the "pussy" my bunch of guys made decades ago when I was young.
It's so simple and can be done anywhere there's dirt. This is best performed on a warm, sunny day when the ground has been heated up. Whatever the size of your hard cock, length and girth, dig a hole a couple of inches deeper and bigger around, wherever you know it'll be a private place. Make mud with the proper amount of water, not getting it too thin, but with a consistency to cause some resistance. And then, fuck. having made sure there's nothing but mud in the hole.
Mud probably is easier to wash off, than, as 6inchwonder pointed out, chafing is to heal. Hopefully you'll pick a spot free of snakes and other unwanted guests!
L