Help converting Fusion Guns

Well, I've got fed up with trying to find old metal Storm Guardians on eBay to pinch the fusion guns from for my Corsairs so I thought I'd have a go at making some of my own (plus, I much prefer plastic models in general).

So far I've found that it's actually really rather easy to carve up a Shuriken Catapult to look like the Storm Guardian fusion gun body, and the tips of Harlequin fusion pistols work rather nicely too.

The bit I'm stuck with is the fuel tank. I've tried a couple of things but everything seems too big/not the right shape.

Any ideas for a bit I might be able to use for that?