Koko,
If you want to extend your mattress "just a bit" you can buy a "CHEAP" mattress pad, roll it up and place it at the head of the mattress. Just pull the mattress down from the head of the bed, stuff the rolled up mattress pad (or even a couple of firm roll pillows) and put a fitted sheet on to cover the whole thing. We've been doing that when we get set up in a park, and just put the roll pillows under the bed when we travel. The only reason I don't leave them in place when travelling is because they do put the mattress very close to the sliding door to the bathroom and I don't want to have the mattress sliding into that door and possibly doing any damage to the hardware.
Just a consideration, but you'd be surprised how you never feel what's under your pillow at the head of the bed. It's a lot cheaper than a custom mattress !!!!!
