How about using one of those blue plastic water barrels that you see around farms? I'm sure i saw one of the Sammio/Miglia/A352 boys using one?
A quick e-bay throws up loads at between £5 and £10 i.e.
http://www.ebay.co.uk/itm/Blue-plast...-/261564240732
The curvature is very good for inner wheel arches and the leftover bits allow for lots of tinkering materials!