I have also used a hollow core luan door, with folding table legs screwed on from underneath. They work pretty well, but the main problem is that over time, or if you leave any weight on it, it will sag, unless you add a center support, or add lengthwise stringers underneath it.

Lately, I have been using a rolling workshop table from Home Depot, and I LOVE it. I was worried about the top flatness, and was ready to take it back if it wasn't good, but it was quite flat in both directions, checked with my 6' level, the top is substantially solid, and I can easily move it aside or across my workshop on its wheels, if needed. 2' x 5' is plenty big for a building surface for me.

After having tried a new method suggested by the instructions in a kit that I recently built, not liking the results at all, I am going back to the old way of adding ceiling tiles to the top (for push pins) for my next build. The suggested method that I didn't like was using CA, instead of pins, and tack-guiling parts to wax paper directly on top of the solid top. I will never do that utter nonsense again! It doesn't hold anything down and you can count on parts being cracked/split/etc., when you try to cut or pop loose the CA tack joints.

Also, when I could get by with a small work surface, I built a frame that would accept one rectangular ceiling tile. I just set it on 2 sawhorses in my shop, and it worked extremely well for planes up to .40 size, if I got creative when joining wing halves.