A buddy of mine carries base board…some people call it moulding…? It’s just cheap wood from Lowe’s. It’s 3″ x 96″. He cuts it into 3′, 4′ or 5′ sections…and uses it for those hard to get places. It’s simply used instead of tape.
As far as vertically goes…I spray arrows and floor numbers onto a parking garage wall semi-regularly. If that’s what you mean…I just use the same ol’ stencil that I’d use on the ground…especially if there are 400. Cleaning is easier.
I like the 125 Mil. It stands up…and I mean literally stands UP and doesn’t bend and hang down when I’m trying to spray it.
Low Density Polyethylene Plastic is best.
That said…I’m not too sure I have it right…what do you mean by “400”…? Is it 400 stencils on walls?
Last…these tend to be smaller than the parking lot stencils. I use brown jersey gloves when I hold the stencil in place.
Anyhow…I hope that helps somehow.