The reason I recommended a hollow core door in TinmanJones' (Don) other kit thread is primarily because a hollow core door is a reasonable combination of two things: very low cost while delivering a flat surface.

Having said that, I built all my RC airplanes on an old hospital solid core door that was not only huge and heavy, it was incredibly flat. In spite of whatever type of heavy hardwood it was, I could push T-pins into it - although my fingers would get sore.

I've never used ceiling tiles. My only concern is they wouldn't be flat enough. For me it's all about having a flat, level surface to build the airplane on.

I gave away the hospital door many years ago and I haven't built a kit since. But, if I were to start a kit tomorrow I'd buy a hollow core door. If it started to sag I'd flip it over. If it reached a point where it wasn't level I'd toss it.

Don - I don't know the price of solid core doors. If it's not that much more money maybe you should get a solid core. If you do, when you're at the store go to the level section, "borrow" a long level and have the salesperson open the door box so you can check that the door you're buying is perfectly level and flat.

I'm sure everyone's suggestion will yield positive results for you.

Edit to add: I mix epoxy on Post-it pads, usually with toothpicks. The top Post-it gets peeled off after use, and so on.