Hi, If you have tried all the Mods (since the physical parameters of an enclosure are never going to be the same) it sounds like the enclosure is physically allowing the heel position to extend outside the performance range of the circuit and certain components can have up to 20% tolerance adding further to a problem. This is not a fault in the components it is just what to expect. This is a reason for the Trimmer.
For a quick and easy fix I suggest using something to keep the heel from going back just that “little bit” too far. A rubber foot used for the bottom of pedals comes to mind as something simple and unobtrusive or anything else you can think of to keep the heel from extending that extra centimeter or so outside of the range of the circuit. There is a convenient spot for doing this in the middle of the back once you find the correct height needed and place it in the center of the back that looks completely normal.
I never had issue with crybaby or vox shells however I have heard of some shells that allow just a tiny bit too much extension. If you have a handy “rubber foot or bumper” simply place it in the heel location that will block the treadle from rocking back too far and give it a try to see if that is the cause. If there is another mechanical way to solve inside the shell I would be interested to hear any alternate solution.
This is not a totally uncommon problem and you can see in this video a fellow found a way that he liked to limit the travel of the treadle: