You could certainly build a box to fit up on that ledge that could get you some good sound up into the cabin, it's just a ***** of a place to really get into to do anything. Ideally you want to completely seal off the trunk from the cabin too for best results so whatever isn't your port you'd install some wood (easy) or sheet metal (hard mode) and fill in the gaps as best you can to keep down cancellation from sound wave bouncing back into the back of the trunk.
Perhaps if you determine the precise dimensions you want to work with and tell us how much power you're trying to run someone can chime in with a sub choice that would perform well in 4th order bandpass. 4th order can be made to sound good and it is the best way short of just walling off at the B-pillar to get big sound out of that sort of vehicle. You can likely brute force it and just use a lot of cone area and power in the trunk to get some decent bass in the cabin but then you will need to go to extreme lengths to sound deaden that trunk so that it doesn't turn into the kind of trunk rattling hood-rat mobile that you don't want. Going brute force would likely require pulling the spare tire out and using most of that well below the shelf.
I have seen only one build local using Morrel but that guy seems happy with them. They're expensive enough and well regarded in DIY car and home audio community you should be fine. For more accessible brand names I've always had great results with JBL/Infinity. CDT had a big sale last week and I picked up a few sets of their components on the cheap, I've had good luck with them in the past as well but IDK if they're still running the big blowout sale.
Honestly it's going to be more about speaker location, good aiming and prepping of locations then processing power if you want to really dial in great sound. Pretty much anything from reputable brands after you get out of the entry level lines will be capable of sounding good if you take some care in your install and buy either an outboard DSP or a nice head unit with some good EQ, crossover, and time alignment type features built in.