Plastic paint

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Mine is "satin" but I think they do a shiny version.

This is what the tin says:

SUITABLE SURFACES
Exterior wood, weathered plastic, metal,
concrete brick stone, PVCu and plastisol. Notsuitable for use
on decking, copper, polished concrete/concrete floors or
common fletton bricks/engineering bricks, If in doubt please
call 0330 024 0298 or visit sadolin.co.uk, or referto the
technical datasheet.

Could you possibly paper it with vinyl wallpaper?

We got an oddly bath panel online a few years ago.
Do you know the make?
(It may still have the label underneath?)

Otherwise (if it's just the outside of the bath) have a look at Supadec - that paints over lots of plastics.

This sort of thing, only this is a photo of a commercial panel at about £400!

We have the same problem. The bath was in the house when we bought it and the rest of the bathroom is fine.
After a lot of research we are going to replace the panel with vertical plastic weatherboarding which will be able to take into account the curve. DH is very skilled at DIY, but it probably isn’t a solution for everyone.
We couldn’t find a satisfactory paint option, sorry.