I did make a gasket for an integrated thermostat housing (plastic so) bolting on aluminium engine. Using red silicone. I tightened right after applying and filled the circuit 2 hours later and went for a drive. It's been 7 months and 7000 miles and no leak so far!
‘01 Peugeot Partner 1.9 D, 5W40 A3/B4, 262000 Km, running on WVO
Wife's '19 Suzuki Vitara 1.0 Boosterjet (TGDI), FACTORY FILL, 6000 Km
Backup's ‘01 Toyota RAV4 3-dr 2.0 VVTi, 5W40 A3/B4, 158000 Km