I have a kinesis advantage. There are some things I don't like about it, but I doubt it would be practical for me to make my own keyboard with a bowl shape like kinesis or maltron keyboards. And even if I could figure it out, they've probably done research into exactly what kind of curvature works well for the average hand, so whatever I made would be marginally less ergonomic.
Now, if you are not happy with the curvature things get more complicated and I'm not aware of any projects that did a custom housing. That being said I think it is not unrealistic. The biggest problem is to create a curved cover with holes for the key switches. All the switches are mounted on flexible PCBs which should adapt to your new case as long as the curvature is not too crazy. So basically all you need to customize is the top part of the housing, the rest can stay the same.
[1] https://michael.stapelberg.ch/posts/2013-03-21-kinesis_custo...