If the tube is “slightly” ovalized, you can remove material from the clamp to match it. I’ve done this on a mtb I have. The downside is the lack of rotational adjustment, so you have to get it right.

For a road bike, you can get a clamp for braze-on front derailleurs and do the same, or get one a larger diameter than the tube diameter and sand the plastic clamp shim to fit. The advantage of having a braze-on mount is being able to use different front derailleurs, especially important if you ever go to index front shifting.

John