Does it have to be divided? How about a tufted headboard that can just be lifted and set aside for the occassional times you need to access the space behind?

Can you post a picture?

This looks interesting

http://thriftydecorchick.blogspot.com/2013/07/diy-tufted-headboard-tutorial.html

I agree with PhoneLady. Why not have your headboard held against the wall by the weight of your bed without actually attaching it to the bed. Then, you could pull the bed out slightly and push the headboard aside to access the cubby. I use a standard metal frame for my mattress with an unattached headboard with no problem.

Do a search for "french cleats". You can make them or buy them readymade. Just lift your tufted headboard on and off, easy.

Here is a link that might be useful: French Cleats How to make

I love that link, phonelady. At first I was thinking it would have to be divided, but since we don't use that cubby space often, it probably could stay as one large piece to be lifted off when necessary. I'm going to try to build something this weekend. I will post the results!

Good luck!!!!