Just cut a piece of plywood to fit. Paint it if you want to.

I would put some sort of fabric/rubber on the underside of the plywood, so as not to scratch/wear the finish on the tub

I would pad it and cover it with vinyl or Sunbrella fabric.

This idea to intrigued me so I googled.

I've read a lot of biographies about the Duke and Duchess of Windsor. One of the more interesting facts I learned was that the Duke (formerly King Edward VIII who abdicated the British throne to marry Wallis Simpson) was incredibly disorganized and always had papers strewn everywhere. In fact, the tub in his private bath at their Paris villa was filled with all sorts of paperwork, mail, photos, etc. It drove the Duchess crazy, so his solution was to have a mahogany top cut to cover it over. So I chuckled when I saw this thread, because all I could think of was him!

to me a bathroom is a bathroom for a couple different functions, why would anyone want a sofa.

I was to a couple clubs in the 90s that had huge lounges in the restrooms. One had a bar in the restroom. Needless to say it was a unisex restroom. I think a cover could be practical for a little-use bathtub.

Maybe if your ‘powder’ room is a full bath and you want to hide the tub and shower since they’re never used?

I worked on a project where the lady had turned her tub into a sofa, like above. In the renovation, she removed it entirely and replaced it with a real sofa with Sunbrella fabric. It was a great South facing room with a window out to the garden off of the master, so it was almost a sunroom feeling.

My favorite part of my master bathroom, which has no tub, is a cushioned bench. I spend far more time on that than I would a tub. It is great for dressing, pedicures, lotioning, etc. if the tub is non used it might as well function for comfort and if possible to hinge it it would also function as storage. I think it is a great idea.

Just remember to pour a little water down the drain once in a while so the trap doesn't dry out.