If you really want a shower bench like that, there are several articles that show people building them after the fact (like you) using cinder blocks, bricks, and mortar to get the shape and then finishing it off.

In fact, if I remember correctly, this was discussed in the Kerdi shower book.

However, I will agree with most everybody else. A corner bench like that is basically useless except for storing more shampoo bottles. Number one regret of my current shower build.

What's your waterproofing plan? Hope it is more than just cement board...

YMMV