Need a small breaker panel to finish Inverter Replacement

You don't need a main. Your main between your power cord and inverter protects on shore power. Your inverter shut down circuits protect on inverter power. Is your inverter even capable of enough power to trip a 30A breaker?
If you really want one then get a 2 space breaker panel like the many that have been shown in this thread, install a "main" 30 in one slot and a tandem 20 in the other. Then wire the output of the main to the input lug of the other buss bar holding the tandem.