Attaching scenery to L girder

That's the way to do it. You can even do it in a modular construction if you put fascia boards on the front and rear to protect the scenery edges.

use a pliable material like masonite as a fascia and have it go 1/2" to 1" above to help the ground cover from falling on the floor as well as give your layout a nice finished touch

Yeah, a good fast easy way to build a scenery framework is with a web of corrugated cardboard pieces stuck together with a hot glue gun. On my HO layout I covered it with brown paper from shopping bags, then gradually built up on that with patching plaster for a thin, lightweight scenery shell. Once you actually get started on it, the work can go fairly quick.

Having a basic scenery shell on a layout is such a huge improvement over open benchwork. And it doesn't have to be 'perfect', it can always be changed or improved later if the mood strikes you.

Right on, Ace!  I've added so much to my scenery that it'll be like an archaeological dig to take it apart when the time comes.