Re scenery on L-girder benchwork... Mine is L-girder (quick, strong, uses less wood), but I haven't yet done any scenery on it. However, I laid plywood over it, so there are no holes to fill unless I create them. If you don't go the plywood route, it seems like gaps can be filled with thin plywood or rigid foam panels.
My plan has been to build raised areas using foam, and cut openings where depressions are needed. Sadly, due to inertia I can't report on how well that is working out.
I do like the idea of elevation changes that L-Girder allows and it is good to hear your experience has been positive, especially the quick and strong part. Am leaning towards foam for mountains and hills with maybe using it as a base covering so it can be "dugout" for small streams etc.