Not sure I'd recommend the Toyota under seat sub. It was, well, a bit pointless to be honest.

To get bass (ideally) you need a big speaker in a big cabinet. The best I had was a 10 inch driver behind the back seats using the boot as a cabinet.
You'll never get that in the MR2 'cos there just isn't a big enough volume anywhere.

The theory of porting is that you take the pressure wave off the back of the driver, pass it through a long tube such that it re-appears at the front back in phase. You can only do that at one frequency so you normally organise it to reenforce the sound just where the bass is rolling off. Again, you haven't got the room so I wouldn't worry about it.

The only other thing you could do (and you'll have already have done this) is turn up the bass to compensate for the limitations. But then you'll be limited because you'll run out of travel in the 4 inch drivers at the front. Unless you filter the bass out of the front drivers and let the back ones do the work...

Anyway, If it sounds better then you've done a grand job. Tea and medals well deserved.

James