I don't understand, Greg. If your hub opening is 56.2mm, just glue 'em in with a few spots of clear silicone adhesive. Use a penknife to remove when needed.
If you're saying your outer hub opening is 58mm(the same diameter as the outside measure of these caps), then you WILL need something to fill the 3mm diameter gap.
I ran into this with the 58mm bore on my CD3s. There is no lip within the bore opening for a wheelcap tang to grab. So I spent a Saturday morning at the grocery store, until I found a bottle cap that gave a good tight fit.
I slap on a red Abarth sticker, add a bit of silicone, and pound 'em into the opening.
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