jtallen83,
The MCTAR-30HD buffer tube sling adapter from Midwest Industries arrived today.
It comes with two sets of screws, long and short so it can be fitted to the two different tube diameters.
The SIG PM-400 has the larger tube so the adapter appears too small at first, but if you use the long screws to get it started the adapter expands to fit without distorting the buffer tube.
Works like a charm. The brace is going to stay put now.
Thanks for the suggestion shibi.