Here's what I've done for rib removal:

Use a matte knife/utility knife:

Begin by carving a notch toward every intersection of the ribs at an angle . . .

. . . alternately cut straight down along each intersection . . .

. . . keep at it until you leave just a single square column standing, but it'll break off at the bottom before you really get down that far . . .

. . . and you'll have four V notched ribs that are no longer connected right down to the base of the intersection where the column disappeared . . .

Then all you need to do is score the joint between each side of each rib where it meets the bottom (of the top in your case) . . .

. . . then wiggle each one side to side until fatigue breaks it off flush with the bottom (inside of the top) and you're done.

Then again you could just grind the schiznit outta the thing with your rotary tool . . . I've done both. [}] ]'>

When doing a lid like that, especially the front side with no protrusions. You can make a jig to keep a full size router within limits and supported at just the right height and blow all the ABS out of there in a couple of minutes, making it a bit deeper than the existing surface. [] ]'>