I had the base SS system in my first Air (leased), but didn't have time to swap out anything due to competing life priorities. What I did understand, though, is that base SS does not have the front kickpanel woofers (Lucid calls them subwoofers). All of the bass comes from the rear deck woofer. The cutout in the sheet metal panel is probably still there, but you wont have the OEM mounting solution to use as a template - the panel is not flat. The thread I referenced above has very good photos of this and the mounting rings developed for the aftermarket midbass. You'll definitely need a custom mounting solution here and you'll likely need to run speaker wire as I doubt the harness will be there.
As far as what brand to use, same advice on dimensions. In this case, since the OEM drivers are 8s, my focus would be on depth. You will be constrained on both the back (kick panel depth) and front (distance to the dash) sides. Utopias are relatively shallow for their size. A solid contender at about a quarter of the price would be the Helix Ci5 S200FM.