I painted the wood facia black and attached poster board along the side of the table. I use long clear pushpins to attach the poster board to the facia. This way I can remove an individual section ( several sections if needed ) of poster board when I need to crawl under the layout. Originally I had planned to use gator board instead of poster board, however, when I check the much higher price of gator board, I had no problem making the choice for the much less expensive poster board. The poster board has been working well for me since 2019.
I used the color black because theatrically black allows the eye to be drawn to focus on the layout itself. Being congruent with the black poster board and helping to keep eyes focused on the layout, I also use a gray/black set of thick rubber mats along the isle way which feel comfy on my feet and knees when I'm working on the layout. These mats also allow visitor's feet to be comfortable as well.
A side note: When my brother built the benchwork, he suggested having the facia raised 1 inch above the top of the homosote. ( The homasote rests on a plywood base.) This way there are no scenery ( ground cover, etc.) spills off the sides of the layout and onto the floor. That was a great suggestion and it works well! Thank you Bro!! 
