Single SA8v2 sealed on ~700RMS

[zako 10+ year member]

Pre-fab boxes should work fine for a sealed subwoofer. All you need to get right is the volume, after subwoofer displacement. After buying, you can adjust it with blocks of wood or poly-fill. If the OP wants sealed box sub, I'd recommend to look into Dayton Reference subwoofers. They have reputation for good SQ in either small sealed or ported box design. The 10 inch version of Dayton HO has 4ohm impedance. The 12 inch has a 4ohm SVC and DVC versions.