Just thought I would add my 2 penneths worth. I'm no good at DIY and would get a friend to make it, but here is what I designed along with the cutting instructions for a single 8 foot x 4 foot sheet of conti/chip board.
Lengths could be extended slightly to take up some of the spare space near the end (possibly 2inches longer.
One thing to ask as well is;- What do you use to water proof the inside of the tank with. I've heard Yacht varnish, but don't think that would be available from B&Q's.
yeah, just make the runners as wide as you like (upto 6 inches each), but no wider than the space left. If you are lucky your local DIY store should be able to cut it to size for you. Just leaving you to put it all together.