Lovatic: I would not attempt to put something under the seat. I suspect that bending to match the contours of the vehicle seat is perfectly OK.
And, an UPDATE:
Oldest kid (my husband's daughter) swung by today and was gracious enough to take my 3 year old to the park. 2nd row of the Suburban was already full of kids, so I tossed her CA65 SE (same as the LX) in the 3rd row, in the center since that's where the top tether anchor is in her model year. And then it hit me: "Hey, hon, grab that new, flat booster-like thing out of the trunk of the Honda, let's see if it fits back here." (Yeah, my husband isn't about to know what an Incognito is).
And... IT FITS! Easily. I would suspect you should be able to put two Incognitos and almost any non-monstrous harnessed seat in the 3rd row. I make no promises about Frontiers or Nautiluses, but it should work with almost anything else.
I swear, this booster is the holy grail of 3-across. Bubble bums for smaller boostered kids, Incognitos for big kids, and suddenly even the third row of the Suburban becomes useful.
Except in mine, since someone brilliant at GM decided not to put a top tether anchor in the center of the 3rd row seat in MY2000. Oh well.
