A few points, which can be confirmed by looking at section 6.7.3 of the
US National Competition Rules:

1. A handheld GPS capable of recording altitude will provide acceptable
documentation at any US National or Regional contest.

2. An IGC secure flight recorder is required for primary documentation
for scoring that will count towards US team selection, but that doesn't
prevent you from using a non-secure recorder for secondary
documentation, if the primary recorder fails in flight.

I'd suggest a cheap handheld GPS with altitude recording capability as a
backup device, with a simple SPDT switch to allow selection of the
primary or secondary NMEA data source for the LNAV. KISS...

Marc