If I'm using a rigid tank (which is a PITA) it goes at the back as I have a Pelicase secured to the middle section (unlike the pic below I have the case turned round the other way now as it sits better and a little lower in the 'V').
I use custom made lengths of 8mm shockcord and various fittings - you can get all the bits on eBay.
But it works 100 times better in this boat with a flexi bladder, much more room to move about in the boat and the bouncy fuel tank moving about.
 - interesting idea from Nasher there, you could I suppose drop the bladder into the 'V' but it would need a thin sheet of ply over the tank and under the rubber matting. Advantage would be the flat floor but as Aerotec users know the sloped floor opposite your feet works really well for bracing and comfort.
More pics here: