I've been doing the same experimenting lately. I have an Amazon chinese gauge, simple black box with backlit lcd, and a k-type thermocouple ring.
It will not read properly off of my bike's electricity. It definitely wants a stable 12v power source. Right now I have an extra battery shoved in my seat bucket (itzaskooter) to power it and that's giving I consistent readings. Next step will be building it scavanging a small 12v rechargeable pack I can tuck into the legshield or something.
I just ordered a two probe handheld temp meter off of Amazon before figuring out that my current gauge wanted cleaner power. My plan was to just run that in a cellphone bracket. I've got multiple bikes though, so it'll get used.