Understand that these mags were really designed to be one-time-use items for the military. They were intended to be fully loaded with 30 rounds in the extended position, to minimize spring pressure. This would allow for long-term storage of the loaded mags. Then, in an emergency, the extensions could be closed and the mags would be ready to go.
The problem is that the mags were not designed to be extended and closed a lot (only once, really), and the locking tabs will quickly break. Also, the mags don't really have enough spring pressure to work reliably with 45 rounds in the open position, because spring pressure is too weak feeding the last few rounds.
Given that USGI mags can be left fully loaded without wear or damage, there is little reason to choose Thermolds of any type for this purpose.
-Troy