SVS Subwoofer Isolation feet compared to standard feet. Looked promising, but made little to no difference in rattling on mantle/walls/etc. : r/hometheater
SubCone Isolation Feet for Subwoofers and Medium/Large Speakers - 4PC
4X DIY Rubber Speaker AMP CD Isolation Damper Feet Stand Subwoofer Pad Foot Tool | eBay