Just replying as an update now that it's summer again.
The new control cables are a big help. I do still have a slight leak on the pilots side yet, so I may have to resort to adding something thin for a film on the flapper door to completely get it to seal the hot air out. But without those locking cables with the pushbutton, it would be a losing battle keeping the heat out.
120+ hours on it now and it's been going great.