Adding Subs with a Custom Enclosure

Ill start off by saying and I am a first time prius owner (my '10 is on its way). I got the Prius II because I felt the extra thousand for the III would be better spent putting in a custom system to my liking. Seeing as the '10 is brand new and no one has done anything to it yet, I figured I would start off slow and easy. I am thinking about getting a custom enclosure made to fit in the under-floor storage in the rear. I want to get it made so that it can be removed in case I need to get to the spare tire, but the only problem is that I do not have my car to look at the size area I would be working with. I have 2 JL Audio 8w1's id be putting in the enclosure. Do any of you think that this is do-able? I would be getting it done by a professional car audio installation shop (not best buy, but an actual shop), do you think they would have any problems installing it, I am not trying to have my new car completely messed up--- (horrible experience with Best Buy). Thank you!