Hi, efhjr. We have lots of customers using grungebuster Washers and Extra-Thick grungebuster Washers with applications like this. Two regular grungebuster Washers and a PTFE washer would give you only 3.6mm thickness, though. Three regular grungebuster washers (1.6mm thick each) and a PTFE washer (.4mm) would give you a thickness of 5.2mm, which would compress to about 5mm. (Likewise, one Extra-Thick grungebuster Washer (3.2mm thick), one regular, and one PTFE would give you 5.2mm thickness.)
Steve Herbelin
Herbie's Audio Lab