2pcs/set 3.5 Inch Universal Hard Drive Mounting Bracket Adapter For 5.25 Inch Bay - 3.5 To 5.25 Hdd Drive Adapter Bracket - Pc Hardware Cables & Adapters - AliExpress
3.5" to 5.25" drive bay backet adapter by drug123 - Thingiverse
NeweggBusiness - ORICO Aluminum 5.25 inch to 2.5 or 3.5 Inch Internal Hard Disk Drive Mounting Kit with Screws and Shock Absorption Rubber Washer- Black (AC52535-1S-V1)