I don't really have the stock or tools to make the spacer, so it seems much easier to me to just create a groove in the backhead. The backhead is about 1/8 thick, I need to make the groove about 1/16 deep, which isn't too bad.
The instructions say that I should screw the backhead on, but there aren't any good positions for screws - I'm thinking that I'll ultimately try to solder it on.