I'll have a think Keith, but as an alternative, what about a rolled up piece of Neoprene? Almost like a big mouse pad.
I'd find a piece the correct size then sew velcro around 2 points on the middle to secure it in its rolled up position. And somewhere near the top attach a metal loop to attach it to your belt. ven just use more velcro to attach it to your belt?! I love spending money on fancy stuff, but think this could almost be better than a proper unhooking mat. Less bulky, easier to clean etc. In fact, now I mention it I might even start making a prototype?!...
this is a tricky one as most carp anglers ie me don't mind bulky mats due to size of fish. When I am out stalking I use a mate by Chub it rolls up into a size you can strap to a small pack plus not that expensive.
I use a black rubble sack as a quick weighing sling (not used enough) and it offers good protection against barnacles etc, without pulling off loads of slime. Doesn't offer much padding but there again wrasse and bass and pollack are pretty robust so long as you don't go dropping them.
A foam mat would worry me as it is bound to end up getting wet, absorbing water and getting heavier.
Ben's idea of a neoprene mat is a good one, but the issue with any material like lined neoprene is that it might remove some of that slime. I have got some of the titanium lined stuff (courtesy of Mr Esclapez).
Deployment is going to be the issue if you are fishing alone, presumably you'll need to lay out the mat with the fish in a boga or in a net in the other hand I guess. Thinking of ways of rolling it up involving shock cord and toggles as I write this.......
Another thing is that the fish can be very feisty especially if you get them in fresh so do you need something that contains them, or would it be easier just to put the mat in a depression in the rocks/rockpool?
Mind you I haven't seen a damaged wrasse in any photo's yet, which is great.
I would say the standard small foam carp mats get very waterlogged and smelly etc. wouldn't like to use one for bassing.
I think the neoprene idea is great, either a sheet of neoprene or a standard camping mat cut down, in a rubble sack (wouldn't want to use black as will get very warm) heated with a heat gun and sealed would make a great bass mat.
either that or a sheet of sticky plastic glued down to one side would give you the protection for the fish and not take off any slime but the grip from the neoprene on the rocks on the other...