I did exactly as fsm1000 recommended and it works well. I used a typical smooth sided hollow door, and plan on affixing it with industrial velcro (also at Lowe's) to keep it from sliding off the folding-leg table top. I'm going to try the velcro idea because I drilled holes through the door to route turnout and powerpack wires and would like access to them just in case.
As far as smooth door construction goes...if you ever need to cut some of the door off, you'll be cutting through the strength of the door since there's only wood around the edges (normally there's cardboard strips in the door's center), but grab a yardstick from Lowe's too because it is the perfect height to sandwich between the door faces if you cut off the rigid wood framing. I had to cut 18" off the bottom of my door (wish I had that little bit extra back for more scenery/track) and glued the yardstick (cut length to size) between the top and bottom layer of the door face. I've had no problem with twisting or warpage and the yardstick is a perfect fit.
Lastly, consider buying 1" foam insulation board (or whatever size you'd like) too and glueing it to the door top. That will give you some options on creating ditches, rivers and ponds on the layout. That's one thing I wish I would have thought of before starting my layout.