I have only had leaks in new 4x5" holders, not old ones. I have not had any problem with any 9x12cm holder (all used)?
The 5x7" holder problem is not a leak - it's a bulge in the middle of the image area which pushes the film against the slide so that it gets scratched.

I think the solar eclipse - image should be about the toughest bellows test of all: 20 minutes with the dark slide out, lens pointing into the sun. I used a 9x12 holder for that...