Richard I am confused as it looks like having the correct insert for your Router plate is what you need. I assume it would be a Porter Cable size bushing opening.
I think you're right in that I'm going to have to make my own insert, oh to be back in an engineering workshop where I could have made one in short time on a lathe.
The insert in the router table I have now is 94mm in diameter and has three screws to locate and three more grub screws to help level it to the router surface.
I've attached an image showing the insert.
I need an insert that's 94mm in diameter and takes the Leigh guide bush which is 35mm diameter (1 3/8").
I've only found out recently that every make of router has different fittings in respect of the size of inserts; that's crazy, they all need to be the same size (full size routers is a given here) which cuts down on confusion and cost.
The world is madder than I thought possible.....