Backfeed breaker use
You can use a regular BR2xx (two pole) breaker for this

As you can see in the picture below just to the right of the breaker's UL mark, Eaton BR two-pole breakers have the screwhole required by the BR backfeed retainer kit as a stock feature of the breaker frame:



As a result, you can simply use a two-pole Eaton BR breaker of the appropriate amp rating for this, given your current hardware constraints.  (It won't be 100% Code unless you pull the N-G bond out of your genset, but it won't zap any linesmen, at least!)