I dug out my spare tailgate to make an example out of it. Poor tailgate
So you can see that once the carpet is pulled off there is a very easy cut line to follow.

Drill a starter hole.

You can see the divots where the 3 ribs are spot welded in with 3 spot welds each. With a normal length jigsaw blade you can cut right through the ribs if you go slow.

Cut along the line making sure to remove the entire raised portion.

There is one section that you can not cut with the jig saw, if you do you will cut into the latch. I just use a cutoff wheel for that little bit, or you could block up the jig saw a bit, or use a shorter blade.
If you remove the inspection cover you can see the problem area beforehand.

A shot of the interference once the center has been removed.

Once the center is pulled out, remove the ends of the ribs by tweaking them back and forth until the spot weld breaks.