Cheapskate water tank liner

[snowbird30ds]

I have an aluminium water tank and would like to line it with a plastic liner without costing a fortune, just had a quote from duratank for a liner and nearly fell over, I am happy to cut the top out and make an access panel to do it through and modify so it bolts back in place again but I can't just chuck in any shape liner as I want the level gauge to work again without fold of plastic fouling it.
Any ideas of material to use and adhesives to work with, I want to be able to drink from it so nothing nasty.
It's more or less a rectangle but the bottom vee's down to the outlet.

 

[neil_s]

I've just installed a Vetus flexible tank. It's made of really tough reinforced food grade PVC (bit like lorry tarp) and all the seams are welded. The Plastimo ones are polythene in a fabric protective sleeve. They do sell the polythene bladders separately, though - would one of those fit your tank? I guess if you used an oversize one, you wouldn't need the sleeve.