I think what Tom said, the O-rings, is technically the only 100% correct method as Sealing locknuts and myers hubs are technically only listed for rigid/IMC I believe. IIRC those O-rings are not available above 2" however. Another option that is often proposed on here is a myers hub -> RGS nipple -> PVC FA. Personally, in practice, I would use a regular TA at the bottom of boxes, and a myers hub on the sides and top. (Pretty much I avoid PVC above ground, other than a vertical meter riser, so I rarely run into this issue).