You need to chamfer the back of the spacer to allow the profile of the hub to enter the spacer,this will not weaken the spacer at all as you only need to take approx 5mm out 
+1, I think 5 mm is more than enough
Edit.
Fck me you never squeeze up something like a hub spacer

If it don't fit flat by hand dont stick it on
Non hub centric spacers or HC spacers with poor location spigot into the wheel are full of fail