Choosing the 'mm' for a hotshoe-mount viewfinder?

I'm looking for a hotshoe-mount viewfinder for a fixed-lens Sigma camera - 24.2 mm focal length, crop factor 1.7 [sic].

I suspect that a "24mm" viewfinder may be incorrect for that camera.

Any suggestions?

Indeed a 40 or 42-mm viewfinder would be preferable

Thanks Marc.

I did find a 40mm on ebay but it was from a supplier in China with a pretty long delivery time.

So currently, I'm looking at a fancy Japanese 35mm job:

It's 35mm, so I would have to pretend it's a "Sports Finder" a la SD9/10