OK, you pretty much have to plug that hole for the sub to work properly. First thing I would do is check with the manufacturer and see if they are willing to sell you one of the plate amps that normally comes with the sub for a price you’re comfortable with. That will be the least amount of physical work on your part. If not, it becomes a question of how good your carpentry skills are and if you have proper tools and workspace.
Measure the current cabinet opening. You can search for a plate amp that comes close to or matches one of the dimensions. I had to do this when the original amp on my Dayton sub kit failed and I replaced it. I used some plywood I had lying around, the new plate amp, and lots of screws and wood glue to plug that hole. It doesn’t look great but it works:
That’s gonna be labor intensive and you need to be careful not have direct wood-to-wood contact without some kind of damping like glue or weather stripping between or the cabinet will likely rattle.
You could just have a piece of MDF cut to fill the opening and glue and screw it in place, and have an opening cut in that MDF filler to put in a plate-mounted set of speaker binding posts or 1/4" TS jack. This solution will be slightly less labor intensive but also require you to buy an outboard amplifier. A cheaper Crown amp brand new bridged to mono will likely be all the power you realistically need. You could also explore the used professional amp market. Pro amps usually come with lots of power, have a bridged mono option if they’re stereo, but also usually have pretty noisy fans.