Great advice. Only thing missing was how to install the seat belt stopper button.
My Toyota dealer's part's guy didn't know for sure, and didn't guess right. But google found several relevant technical service bulletins: Toyota BO011-01, Honda 93-033, Accura 93-015, Lexus BO005-0, Nissan NTB99-022c (e.g.
http://www.ih8mud.com/tech/tsb/LexusLX450/bo005l01.pdf).
This worked for me: Use a pipe wrench to turn the knob on an adjustable crescent wrench to squeeze the parts together. I even remembered just in time to slide the belt buckle upstream of the button.
Using a vise, or some other way to apply pressure from very-flat very-parallel surfaces should also give a professional looking result.
I just hated the annoyance of reaching and fumbling to find it, and worse still, that my kids might have a reason to not buckle up. Best $1.50 I ever spent.