One very easy way to get a repeatable force is to use known springs and then measure the area inside the bolt holes (e.g. width times height if 4 bolt holes in a rectangular pattern).
With springs, the manufacturer can give you the number of pounds that will be exerted when at X amount of compression. So, you pick the equal springs that (when compressed to the same amount) will give you the PSI you need for your given area.
When you tighten as much as possible, I worry about threads stripping and TEC crushing (if you don't use a alternating pattern of tightening). Good luck!
Nice frost.
