There's the Deckmate, which lets you connect the deck to different monitor mounts. I've seen people put a monitor mount with a swivel arm on the wall above their bed and clip their Deck to it, to play in bed without having to hold the deck up. You could also use the deckmate as a foundation for attaching altermative holding methods to the back. Any method that gives you a good handle on the center back of the deck should be better for touchscreen play. They provide 3d printer files so this can be a nearly free solution if you have a 3d printer.

A popsocket on the back might work as a cheaper alternative to that, but I'd be worried about dropping the deck since I doubt the popsocket is very strong. Should be fine if you're just wanting to use it in your lap though.

You might also be able to use something like this, it's meant to hold a battery pack in a strap on the back of the deck, but you could maybe slide your hand inside the strap to more securely hold the deck from the back.

You could maybe use some kind of tablet rest, but you'll need to make sure the back vents aren't covered.