I know this question is almost 4 years old at the time of me writing this reply, but I'm sure other people have run into this issue. While my situation is a little different, as I'm trying to add overload protection to a cart which uses a power inlet box that you would normally use to hook up a portable generator to a generator transfer switch, it comes down to the same problem of adding an inline breaker.

As pointed out by Retired Master Electrician, over current protection devices can only be installed in an approved enclosure so even though my application isn't likely to be inspected, I can't buy a gang-box mounted breaker because no one seems to make one. While I could buy a panel mount thermal over current breaker an mount it to a blank decora insert, it wouldn't be high current enough for my application.

The correct solution

This is why these load centers with only two breakers exist:
https://www.lowes.com/pd/Square-D-30-Amp-2-Spaces-2-Circuit-Main-Lug-Load-Center/3457972

One of these can be installed inline between your outlets and the main breaker. The outlets can be moved to a new breaker in this tiny load center, and the new lighting circuit can be added to a second breaker, converting the original circuit into a sub panel circuit.

According to Can a circuit connecting to an outlet be branched as a subpanel? you cannot have any outlets upstream of the new sub panel, as a circuit must either be a feeder or a branch circuit, not both, so you must make sure there is nothing else on the circuit ifupstream of where you convertare branching off your new lighting circuit.