On our layout, there is a second level for trolley line. To cover the open space between main and upper decks, I ordered four sections of O-Scale Component Flexi-Wall stone block wall (FL8150). However because there maybe an infrequent need to gain access below the second level to wires, I decided to attach the sections to the upper level with sticky backed Velcro tape available at fabric stores. I pressed the fuzzy to the hook strip then stuck both to the plywood deck. Then I positioned the foam section and pressed it against the Velcro. I then separated the Velcro and placed a few staple through the Velcro to the plywood, and hope the side to the foam is strong enough glue to not peel off. Now I can remove the section(s) if needed. Here is a picture of wall separated shown with Velcro strips.
Part of the upper deck curves around. O-Scale says the wall can bend to 8" radius, but my curve is sharper and I didn't want to snap the wall. And second, The wall has enough stiffness that it would pull away from the weaker Velcro glue. So stood a wall section upright longitudinally and slowly heated it with a heat gun sweeping it back and forth across all areas (allow 4-5 minutes, gloves recommended). Then VERY gently began pressing down on the wall into a curve while still keeping the heat gun sweeping all around. I let it cool (2-3 minutes) while still pressing, then let go and wall retained its curve. I overbent it figuring that the wall would flatten out when pressed against more gentle curve. Attached Velcro as above and done.
Along the lower left side is a hinged swing up table wing that will have yard tracks to meet off a yet to be installed main line turnout. I unscrew the legs (visible as pine 2x2 wood in last picture)to lower wing allowing closer access to track and scenery (that's the theory anyway 
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