I had always planned to use a 20 litre plastic gas tank from one of the marine supply shops. I have one in the workshop, which as it turns out is a tad too tall. That was a miscalculation on my part. But, hey, this is my first design. It's all very well to say "finish a workable design" but there are gotchas at every turn - things I hadn't even considered. Every time I think I have EVERYTHING designed, I discover that either it doesn't work, is unbuildable or something else. I'm TRYING to do as you say - but it's not easy.Good design shouldn't be a lot of afterthoughts cobbled together. There are several basic things a powered aircraft needs, a fuel tank is one of them. Surely you considered the shape, placement, material and construction of this before you got this far. Duncan, before you waste any more effort, materials and money, Stop and finish a workable design and consider how you are going to build it and from what materials.