Other than a hitch mount, which requires you to install a hitch, there aren't really any options. As for putting a sub in that spare tire area, no. Just no. Bad Jake! You can use the storage cubby and install a sub easily there, like I have on mine. You can just get the sub assembly box and factory sub from a parts puller (or pick your own compatible unit) for hardly anything and use that, but I don't think any sub will work unless you have an appropriate amp and the basic Patriot amp doesn't support subs, so far as I know, so that too is a cost and labor factor.