I've tried hylomar and vht copper spray with good results. Got even better with a version of 'three-bond' (can't remember which off the top of my head, but the tube is likely still in the shed since around 2001/2 when I actually used it). I think that 'hondabond' might be repackaged threebond, it anecdotally works as well and might be easier to find in some places but likely not cheap.

Within reason, it's not a great way to reduce the comp ratio, because it increases the quench gap to a such a distance that it no longer 'works' at preventing detonation. Don't get me wrong, if there's no other way, then it's 'the go' but using a really thick head gasket to drop the comp ratio - if you could hypothetically run tight quench clearances and use a dished piston (dish to mirror the chamber shape) you might be safe with a cr of 9.5:1 and a given fuel and boost, but with a thick head gasket and no quench, you might have to go down to 8.5:1 to be safe - quench/flame propogation is a big deal to say the least.

Having said that, if it's detonating or pre-igniting, running the thick gasket and lower than 'optimal' comp ratio is still going to work miles better than destroying and engine!