My bike has one built in, but if I were installing one I would first determine what I wanted it for.
If it was a occational use, I'd put it on a semi-long heavy duty cable under the seat that I could pull out when needed.
If it was going to be used often, I would put it in a easily accessible location, but not drill any plastic and strap it to the handlebar or rack or some other location that is out of the way be easy to get to. That's just my opinion.