How can I upcycle a 12" solid foam king size mattress?

You can use an electric knife to cut foam, but if it's 12" thick that might be very difficult to do. I'd measure the size and mark on each side of the foam, cut through as deeply as possible on one side, then turn the mattress over and cut from the other side. Let the knife do the work with its serrated edges. Whatever you construct, you could use heavy duty garbage bags to protect the foam, perhaps by cutting the bags into 1 piece of plastic and wrapping the foam in it and using duct tape or packing tape to seal well. Then use an outdoor fabric for final covering or a heavy duty flannel backed plastic table cloth. Hope this helps or at least gives you some ideas. i

I've used foam from a deconstructed mattress to re-upholstery but it wasn't quite that deep. If you are going to use it outside just be sure to use a water resistant fabric. You will need to be careful around seams and I would still cover them when you aren't using: https://www.pinterest.com/patboyce84/cigar-boxes-repurposed/

I just picked up an electric carving knife at the thrift store for just this purpose. I recently did a couch cushion replacement job but I had to use a plain ole serated bread knife. I used a sharpie and ruler to mark the cushion. Once you've cut part way through something that thick, I stood mine on edge and pulled a bit apart with one hand against my body and used the knife to cut all the way through.

whatever you cover foam with you'll need to waterproof the fabric use this 303 Fabric Guard, Upholstery Protector, Water and Stain Repellent, 32 fl. oz.

• Restores lost water and stain repellency to factory new levels when applied to clean and dry fabrics by adding a powerful protective unscented coating.
• 303 Fabric Guard will greatly increase water and oil based stain resistance, keeping fabrics dry and maintaining color fastness. Use on upholstery, cushions, umbrellas, life jackets, suede, canvas, and other outdoor fabrics.
• This powerful fabric protectant is safe for all fabric including synthetic and natural fibers.
• 303 Fabric Guard is the only product recommended by and for Sunbrella, as well as other fabric manufacturers to restore lost water repellency to fabrics.
• High quality, premium product. Does not alter fabric color, feel, flammability or breathability. Made in the U.S.A. needs to dry up to 12hrs before it works at its best YOU can cover it using canvas painters tarp(low cost) to make cushion covers https://www.youtube.com/results?search_query=diy+easy+cushion+covers

Thank You everyone. I have one more request. what can i make with this big chunk of foam ???

Larry C