My 3600 also idle around 50-55C. I have used Jonsbo hp400, Jonsbo+Noctua fan, Cryorig c7 copper, C7Cu+120 mm noctua fan. The Cryorig C7 copper+120 mm fan is my best combo so far, and also under volt by 0.10v help cool the cpu a bit. I have Formula Loli 2s case (182 X 114 X 220 mm).