Originally I used 0.3mm copper shims with thermal paste, between the CPU and cooler block. Worked just fine and had no issues with sustained Prime95 loads, but I wanted to see if removing the extra thermal interface would improve anything.Should I adjust for zero air gap or put a little bit PCB pressure /like all PC cpu coolers/?
The janky solution I came up with was to stack a piece of high density foam (cut from some SSD packaging I had) and a bottle cap on the backside of the CPU, and use the bottom case panel to press it down. I cut the foam so there would be 3-5mm of compression when the bottom panel was installed. It's working well so far, maybe a couple degrees cooler after stabilizing in Prime95, and ~1c cooler at idle on my OPNsense box. I'd say it's not worth the trouble if you've already installed shims or something, but if you need a quick and free fix, it can work.
I found that if you use only the foam to bridge the gap between PCB and bottom panel, it compresses too easily and doesn't maintain pressure well once the foam takes a set. That's why I used the bottle cap to fill space.