Cementing to bathroom tile

Need a waterproof method to permanently cement a 15" x 20" x 3/8" rectangle of polypropylene board (same compound as a regular kitchen cutting board) flat against a tile shower wall.
It's to protect the wall from continuuing damage and cracking of the ceramic tile from the angle support for a handicapped person's folding shower seat. It was attached to the tile by lag bolts into 16" center studs at seat level and has the folding wall support under the seat that folds out against the wall that's doing the tile cracking and splintering damage.
TIA Gette
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if you just want it to cover, ge silicone 2. if it's to provide any support, you have to anchor it to the studs in the wall.
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why not just get a seat that wont cause that problem...
randy

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