Finally wrapping up my audio upgrade, maybe this info helps you

I did find search result posts here in Priuschat forum about some folks trying it under the seat. Search 'woofer' in the Gen5 forums and you should get some results that you can skim thru and probably find references to under the seat. It's been awhile since I read thru those but back when I did that, my take from those posts were certain small woofers like the Nano either would not fit or would be extremely tight in the Gen5 Prius. I think I also recall reading of someone else here on Priuschat that did manage to get some thinner smaller woofer under a car seat (may not necessarily have been the Gen5 Prius), but then removed it from there because their passengers didn't like the bass vibrations rumbling they felt while sitting in that seat. I don't think I'd like that either and so because of my overall take on this, I didn't even try that under seat option. Of course opinions differ so YMMV. Therefore, for the Nano and for me, it was either going to be under the cargo area or somewhere above the cargo area.

Here's an example search result that mentions under the seat and the OP's pictures and last posts look like the original under the seat plan was nixed in favor of behind the passenger seat. Gen 5 Prime XSE Premium - Adding a powered subwoofer to the JBL system and need advice | PriusChat