Cool idea.  When I had mine the way I fixed it was grinding down the plastic on the bottom of the ramp under the flap.  The thicker ramps from James rock because they don't break.  The one on funhouse needs a bit of a shave though.  Or your shim ;-)

That's exactly what we've been testing.  We ended up grinding all our Banzai Run ramps because those were just too thick and the flap could not sit flat unless we grinded them.  With FH the flap does sit flat but as described the flap gives.  It is remarkable that this issue took over 3 years before someone brought it to our attention.  So 6 months ago when we found out about the issue we looked at two possible fixes, one is go to with a thicker flap or two is to grind the bottom of the ramps.  With the thicker flap we are concerned that the ball will jump on the entrance lip of the flap.  The other issue with the thicker flap is the thicker metal has a different look than the thinner metal.