I’ll admit that I might not be the right person to answer this question, but I think slabbing a shikishi is… a bit much.
It’s a one of a kind art piece- why encase it in plastic when you could put it in a nice frame and display it for about the same (or lower) price and it would look so, so much better?
Shikishi are beautiful, especially the trim around the edges, and I feel like encasing it would just hide the beautiful artwork and style of the art board.