...do I need a specific one?
You'll either need a brace that's compatible with the design of the seat (Rule 126.96.36.199) or a "seat-width element" that provides a comparable level of protection against catastrophic rearward seat movement (Rule 188.8.131.52). In my car, for example, I have a T-shaped element projecting forward behind the seat. Please note that in my cage the belts do not attach to this T-shaped element; they attach to the cage behind it. The T-shaped element is acting only as a seat brace. I've also seen similar elements that have a wide, flattened U-shape and are welded to the cage at their ends.
1982 MG Metro 1300: IOE 2015 Pacific Northworst GP, Longest Distance 2010 Cd'L Box Wine Country Classic
1980 KV Mini 1: Worst of Show and Fright Pig Supremo 2009 Concours d'Lemons
1978 H Special: Second-Round Elimination 2010 Lemons Pinewood Derby at Sears Pointless
1967 SAAB 96: IOE 2012 Pacific Northworst GP, Organizer's Choice 2022 Hell on Wheels California Rally