I've narrowed down the range of lengths that should work and using the samples on the McMaster site as a guide decided on 60lbs of force per spring.

I ended up with springs featuring



Those numbers include the ball joints, whereas the numbers on the linked page originally posted don't.

Bitt, do you think you might modify the ggb file again to include a spring that can grow/shrink only between 11.85" and 20.12" along with a "lid" line whose hinge is fixed, but and point can be moved between 0 and 65 degrees (or more I suppose, but it'd be nice if there was some marker otherwise at 65).

I don't know what's possible in that software, but it would be nice to be able to move and anchor both ends of the spring so that when the end point of the lid is moved about its arc, the spring line would shrink/grow. That would probably let me pic a few spots to try mounting these things. The most important part being demonstrated by the software being at what positions the spring will be able to allow the lid to open and close properly.