I start this knowing it's most probably not suited for grip (on Capture The Flag to deny enemies perhaps), however it could be a starting point for others to have better ideas, so here is a suggestion of a Tron "light ribbon
" pickup (jet wall sounds cool ^^) :
You know the principle. Applied to GRIP, the exhaust gaz would form a dense "wall" somehow (no clue how), forcing other racers to drive in a restricted space. I don't think cars should "dissolve" just like in Tron, path restriction being enough tactically for us.
Gattling gun could destroy it.
Of course, blue wall if you're under speed boost (based on timer expiration, this means the wall would be longer with a boost on), orange wall to match the normal exhaust gaz color...
Make it the lvl3 of, say, mines if too OP. Or the lvl2 of speed boost (only blue wall then).