Had an idea about the building of the lightsaber in ROTJ, if modified a little (essentially removing the dialogue) it could be done rather convincingly, all you need is a costume some bluescreen and some fancy editing skills. Render a cave backdrop and have the actor portraying luke attempting to assemble the lightsaber from various parts but failing, without dialogue this would consist of him attempting to assemble it and it failing to ignite, he would then remember yoda's talk from ESB, it wouldn't be a force "whisper" he's simply remembering seeing as he must accomplish this himself without help. He then reaches out with his mind and the parts of the saber start to float in front of him and combined with the right music the parts will assemble themselves and luke reaches out for the hilt and it ignites.

The face of the actor would need to be replaced via digital compositing with luke's, it could be done if the scene is set up in such a way that there is not much movement and having his face shrouded for the most part with his hood.

After the saber is built Luke leaves the cave, 3PO and R2 wait at the opening, luke places his hand on R2 and you cut to whatever scene you feel appropriate.