How to fit PVC pipe to threaded-hole in barrel

I'm helping build a rain water catchment system using 9 barrels that come with two holes in the top. We already have an older system in place but water leaks from one of the two holes that come with the barrels. The holes leak because the threads are not the same size as the threads that PVC pipe fittings use.

My brother has a 3D printer and says he can design a part to fit but I'm wondering if anyone's already encountered this problem and what they did.

The coarse thread is called a 2 inch buttress thread adapters are available.