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.
mines built into the bike already..came with it..soo..it's up to you where you want to mount it..just make sure you mount it where it's easy to get to when you need it. as stated..i'd stay away from drilling holes in your plastic...at least save that option for the last case.