That proved to be a project in its own right, as welding was not a skill I had already acquired. It took me some time to get it right. It does look the business now, at least from a distance, although it could do with some paint. Well, looks are not important as the rotisserie works like I thought it would. It makes working on the underside of the tub a lot easier, as well as changing the position of the tub.
I have started removing the undercoat, which I thought would be a tedious job. It is going a lot faster than I anticipated, it will probably take me two full days to get it all off.
The sheet metal appears to be not too bad as far as I can see now. There is some superficial rust in places, but nothing that gets me worried (yet).