How to Plug old In-Wall Condensate Drain?

[Sdradius]

We recently upgraded our HVAC system and installed a condensate pump. The old condensate drain went into the drywall down through the slab and then outside. After cutting off the bend, I have a very short protrusion from the drywall. A 1/2" PVC plug seems like it should be the right size but I can't seem to get the plug inside the pipe and also a cap doesn't fit correctly either. Is this the appropriate way to seal it off, or is there another way?

Thanks! I appreciate any advice.