For the most standard layout... One you could snap back to a standard keyboard when you can, you've got a few options.
Would 20 inch cable between the 2 pieces be large enough? I never considered it for laying flat and I am used to metric so I'm not sure if that's long enough. Surely with some blankets to lift them slightly of the bed you'd be fine. If so, the easiest "switch/smallest learning curve" could be the Kinesis
Then there's a couple off of Ali Express too though might take a bit longer to ship. 1 would be the 2 ymdk split found here. Though it's likely above your budget after buying switches and key caps. And found here. The 2nd needs keycaps. Theymight be on Amazon too? Not sure.