I'm running a circuit through a basement rim joist to a PVC LB connector on the exterior of the house via a short pvc stub glued to the LB connection but just free on the basement rim joist side.

Can I just run romex directly through the unachored side of the stub and connect to the THWN in the LB or must I use an electrical box to protect the basement side?

If inside a box there would still be no way to anchor the stub directly to the box with a connector because then the box would not sit flush to the joist but the box and the LB are anchored and everything is protected. Is there another way?