maybe look at a fibreglass insert then add an insulated lid. we have on in our glass boat built in but added a stainless insert for easy cleaning, 2 bags of ice will last for the whole days fishing and we dont need to do a slurry to keep everything cold. dont even bust the bags open

I remember a mate had this on his boat too ,ended  up  being  heaps cheaper  to buy a mad fish bag and put this in it instead  ,just another  option for you to think about mate  .....

 it'll be pretty expensive if you pay someone to do it. In an ally boat, if it is just an open space, you'll need to build an outer skin, insulate that, then glass it inside. The insulation tends to eat up space, too, you can end up with lesss than you think you'll have. I've done it on a glass boat, was going to do it on an ally, not easy.

The idea of a madfish bag is a good one, I do that on my glass boat, which just has an uninsulated fibreglass tub. Sorta holds ok, but the fish tend to slide around a lot. I have the big madfish, can't fit that in there, so i bought a cheapie Wilson which fitts and works fine, but won't hold near as much as the madfish.

Is your existing space you are thinking of using  already boxed in, or just open space under the deck? 

its just open space under the deck, I’ve currently got a openable hatch there approx 1mtr x .5mtr

i dont mind the idea of a bag but would prefer a more permanent solution

worried I’ll lose too much space building a fibreglass one in there

pretty handy (or more so the fabricator mate of mine) we have modified the rear box & fuel tank already 

was thinking of filling the underdeck space with polystyrene foam for floatation which might double as

insulation for the esky? Thoughts? Then drop an ally skin in.

then some sort of insulated material on the underside of the hatch?

thanks again guys