Medical supply houses, Amazon, and Lowe’s have plastic/aluminum seats/benches for people recovering from surgery that fit in bathtubs for exactly this purpose.

https://www.amazon.com/Drive-Medical-Bath-Bench-Without/dp/B005JIMQL4/ref=asc_df_B005JIMQL4/?tag=hyprod-20&linkCode=df0&hvadid=632050264455&hvpos=&hvnetw=g&hvrand=11810340148234792689&hvpone=&hvptwo=&hvqmt=&hvdev=t&hvdvcmdl=&hvlocint=&hvlocphy=9021237&hvtargid=pla-425062961473&psc=1