Clamshell doesn't quite fit

Do you mean there is a gap between the two halves when assembled on the hub? because that is normal.

if you mean the adaptor will not slide onto your hub i would double check the mesurements with a caliper and possibly even try to get the seller to replace it with the right size.

if the difference is enough that you think it would require more than a light sanding to make it fit you should get a machinist to do it for you.

good point - happens with noobs doing motor mounts too - got to explain to them that anything that clamps around something needs a sufficient gap when tight to be sure it is tight and not just bound together with itself