I bought this slim sub enclosure that fits in the spare tire well. however, theres still a lot of space left and i was wondering if any of you guys have any idea to secure this box in the spare tire well?

 

 

There is no need to secure it with screws or adhesive, all that you can do is extend the surface area of the enclosure to the edges of the spare tire area so it'll maintain its own position. MDF is heavier and will stay in place..

unless youre off-roading lol

 

Wood spacers (aka, scrap wood leftovers) cut to fill the additional empty space on each side.

 

yes, if you look under your box iirc there is a point in which your spare tire bolts too. you could use a longer bolt and then make a hole in your box and secure it through the hole into the threaded part in that. also since your box is sitting on that flat rail that goes across the
spare tire well you could match up the two holes (one on each side of center hole) and secure it the same way. my amp rack in my car has a base board that is secured to that middle rail.
good luck