There's the Wald Woody for example, with a plywood top, and I've seen a few similar ones, but I suggest you get a rack that suits you and DIY the top plate.
I'd probably use 6mm plywood, well varnished after drilling the mounting holes, or 3-4mm aluminium sheet, with the edge nicely smoothed. If you're feeling fancy, 2mm aluminium sheet and a sheet metal bender (try to borrow one) would allow you to fold the edges over for extra stiffness and protection. If there's a lot of overhang, you might want to go slightly thicker, or reinforce the deck. Wood is rather hard-wearing, while aluminium will fail through flexing due to vibrations (I've had a custom mudguard bracket snap on the first day of a long trip and that was aluminium)
To fix it down:
- If you can handle protrusions use U bolts sized to fit the top rails of the rack. U bolts with 8 and 10mm gaps are sold for clamping wires, with a backing plate you'd omit. Example with a fairly stable website but you can find them for far less.
- If you want it easy, just use heavy duty cable ties.
- Fixing dead flat is a bit trickier but pairs of countersunk machine screws in the top deck with either aluminium plates and nuts forming a bridge under each rail you're attaching to, or the backing plates from the U-bolts I linked above, again with nuts.
If you want it long only when stopped, halve the length and put a hinge forwards of the rearmost support, so it unfolds, like this (click for full size):
DIY has the advantage that you can really customise it to fit your load, use, and bike. You could, for example, make it fairly wide and drill holes near the edge for tie-downs, or add a recessed cup holder as in the arms of folding camping chairs - it's really up to you. Just make sure everything is rock-solid and no one will walk into the bike if you want to put a stove on there (or better, cook on the ground).
If you use rather box-like panniers, resting across the top of those may work with a hinge running along the length, but forget that if you use roll-top panniers.
This all assumes bulk rather than really heavy weight, for which you might need to use a stronger construction - but you'd also need a heavy-duty rack in that case.