Found that my alloy wheels have seized on to the axle

[V]

I don't mind using plastic centre caps, I just haven't found any that fit a 4-1/4" hole.

[digger]

A hole size reducer to Jeep ones or something  would be ideal .  I've made my 'holders' from alloy bar(not cheap but you only need a small amount). With that size difference you could  turn some concentric rings or something into it as well!  I'd normally offer to make you a set, if you don't have access to a lathe. However I'm not too well at the mo and you might wait quite a while.

[V]

I have been thinking about having some 4-1/4" diameter solid flat caps turned that have just enough of a conical rim on the inner edge to keep them from flying out. They will go in the wheel from the back like the tin cap style I have now. The difference will be that I can push on them without them crushing.

[digger]

The nice thing about the Jeep fixing method for the cap is that if you are stuck you can pry them out from the outside. This enables plus gas etc to be squirted in after you have removed the Jeep cap from the wheel or out of the reducer ring

 

I've thought about simple flat polished (decorated with rings or plain)or painted caps.If you are going to make some you might consider other fitting from outside methods eg 'O' rings (screwdriver slot perhaps for removal?), interference fit, some type of glue that could be  softened with the right solvent etc.