to seal this gap inside my box....if i glass it, whats the absolute BEST way to approach this, so the glass doesnt ever pop off from the bass over time?

Well obviously tape off the area first. Then lay some mat and resin over it to make your first shell.

Then resin the shell to the box inside and out.

You can also screw the shell to the box and then resin over it. Make sure to add more mat at all the seems.

box is in...cant resin to the outside of the box

Well... Just do the inside then. And instead of multiple strips, lay one big one to make the basic form and then use smaller ones to add rigidity later on.

so staple fleece over the gap, resin, let harden, then add mat? then do the screw thing (after a few layers harden, predrill and screw it to the box)?

will this work... i thought about stretching the fleece, applying a layer or resin, and while its still wet, where the fleece is stapled to the box, screw a 2x4 or something to the box, over the fleece where it meets the box, to sort of sandwich the fleece+resin in between the actual box, and a 2x4 on top of it.

would that help any?

Make sure you overlap the glass onto the wood several inches and sticking won't be a problem. Is the area you are going to glass right up against the sides of the car ? If it is your good to go. Pressure inside the enclosure is going to be pushing out ( i.e. pushing your glass piece right against the side of the car). Personally I would skip the fleece. I would fill in the area with a couple layers before you start overlapping onto the wood