Ok ok.. not sure I want to take credit for inspiring that.
Its a good first attempt, but a bit crude. But you do things to learn. :)
Hot glue will never work. It isn't a strong bond. You have to use proper ABS cement (sparingly!) I would cutout all those extra posts as well. A drill works well for this, as does a chisel.
Again, make sure there is plenty of overlap in the joints, use cement, take your time, mount with screws to chassis, and the box comes out looking good and is very very durable. I have yet to break one in any my vehicles with a great deal of bashing and track abuse over a couple years I've been making these.
Here is what it's supposed to look like:
The underside, with a recess for truggy mudguards. Bottom is ~1/4".
Finally a layer of foam covers the recessed screw holes:
