I have a Bette steel enamel bath similar in shape and dimensions to the one below.
The bath unit is supported by four support feet in the same configuration as the image below. It's set sideways into an alcove, with tiled walls on 3 sides, and the other long edge supported by a (tiled) wooden frame. I'm unsure if there are supports bolting the other sides to the walls.
I've noted the two bath feet next to the long edge of the wall are getting quite rusty.
I would usually remove the tiling from the support frame to install new feet, but the tiles are cemented solidly in place, and it would be a tough job to disassemble and reassemble them.
There is a 50 cm x 50 cm access panel in the tiled support frame that I can use to access the underside (directly next to the bath waste pipe connections) so my options are limited.
I was thinking of propping up the underside with several layers of 50 cm x 50 cm carpeting or thick fabric right under the middle of the tub. Another option is to put in supportive wooden beams of similar size, but the problem then is how to align them with the curvature of the underside.
I'd be very grateful for any suggestions that you may have.