A couple of years back I bought myself a metal workbench and topped it off with a nice flat piece of pine. I started on a kit (a SIG Seniorita) but didn't get far with it.

Since then I've mostly used the bench as a work area for assembling ARF's.

I'm thinking of building a couple of kits, but the worktop is no longer as flat as it used to be. I haven't got the room to accommodate a hollow door or something similar so I've been looking at the portable building boards on offer.

Here are the ones I've seen so far:
Great Planes Pro board
Guillows balsa board
and last, but not least, the Porta-board from Retrop Industries (no web site).

Has anyone used any of these? Any opinions?