If you just mean round shape instead of boxy, check out the e-power. It looks like an ego except bigger. It uses the 41650 batteries and at less than $50, it's a good value. The spring can be extended to use the 41500 battery if you're already using that battery in other equipment. Another mod that looks similar it the noego. Do a forum search if you want to check it out.
If you mean really small the rough stack might suit you.