Thanks for the suggestions.
To be more detailed, the device will have an Arduino Nano embedded (why, instead of just a bare chip? Because I have a dozen that cost me little more than $1USD each, and I have a large supply of header pins). So regulation isn't super critical. The problem (and why it needs an independent power source) is that is needs to drive more than can be provided from the on-board 5V source (but less than 500ma total).
I did forget one other possible option, instead of a linear regulator, I do have a whole bunch of small buck converter modules capable of sourcing sufficient current.
My thoughts on my options: (also I failed to mention directly that this project is for my exclusive us - I'm not making it to hand off to others to use, so any screw ups with plugging in the wrong power or whatever are strictly on me)
1. A USB power connector could be confusing, since there needs to be a second USB connected to the Arduino So while the easiest way to get 5V/up to 500ma in the thing might be a USB connector, there is this drawback.
2. Feeding 5V in via a barrel connector - I'd still need to clamp it in case a higher voltage supply were used. And protect against a backwards adapter being used, which will drop the voltage slightly.
3. Feeding in 9-12VDC and regulating it myself - remembering I have the buck converters, I think this is probably my best bet. Even using a simple diode reverse polarity protection (not even low forward voltage types like a Schottky), the input will be high enough for the buck, and the max input can actually go well above 12V and be good - these have a max input of 40V. I did test them and they have decent output without a ton of ripple and they hold up a 5V output down to a 6V or so input
I'm trying to do this with what I have on hand, which unfortunately isn't a whole lot, but it does appear that option 3 is the best way to go as it is the most foolproof (I need all the help i can get
) and I have the required parts.
Thanks everyone for the advice. Now to complete the design and start building.