Have you seen this thread - http://www.talkphotography.co.uk/forums/showthread.php?t=41904? I did actually go out looking for suitable bowls yesterday but didn't find anything I liked and I figured by the time I'd got hold of suitable diffusion material and put it together I'd still have something that would not pass professional judgement (I could hardly turn up to photograph a wedding with a plastic mixing bowl on my camera) and I'd have a fair bit of faffing about to do. So I just plumped for the eBay solution for a modest premium over the DIY version, plus the eBay solution folds flat for safe storage.
I would make one myself, seems pretty easy