I have both the Vamo V5 and the KSD KMax - they both have 18350 modes, which shortens them a good bit. They can be found for around $30-40$ and they both have decent performance, but I'd probably choose the Vamo V5 over the KMax - I prefer the 3 button setup of the Vamo vs the 1 button setup of the KMax, and the Vamo has better screen (oled vs led).
Also the Innokin iTaste MVP 2.0 is a fairly small box mod that works really well and gets great reviews - has a built-in battery, so no need to purchase batteries, as you would with the Vamo and KMax.
For matching with the Aspire Nautilus Mini, though, the iTaste VV or Vision Spinner might look better.