fasten tanks to diorama

Unattached tanks could present a problem during judging since the judges will be looking to see if your tracks are all in contact with the ground. If they're on a level, paved road AND you have perfectly flat running gear, you should be good to go. Also, unattached vehiclels wil be more difficult to make look like they're convincingly part of the scene.
If you HAVE to remove them after the show, about the only thing to do is leave them unattached, indicate to the judges they aren't attached and hope for the best. Any adhessive you use will damage either your tanks or your base (or both) as you try to remove the vehicles.

Ratbat -

I use thin wire twist ties (like what come on your loaf of bread) through either the road wheels or the under hull (depending on where it wont be seen). Take a look at how kid's toys are packaged nowadays with all that wire passed through a carboard backing holding the parts in place.

Just get your tank situated, drill some 1/16" holes through the diorama base and pass the wire throught. The wire's twist will be on the bottom of the base - you have have to chisel out a recess to hide it so the base sits still. Make sure you use small wire and if you scavenge wire fron the kitched strip off the plastic coating as it will be too thick and noticeable.

After everything is mounted I usually go back and add a tuck of grass, some rubble, or whatever to help hide the wire. A couple small chunks of brick doesn't look out of place under the tank and can break the wire's line enough so that it's not noticed.

ajlafleche and Mark_Henry:
Thanks for the info! I'll work on it!!
rat bat

I fasten mine with a wood screw through the bottom of the tank and into the base. Simply drill a hole in the hull bottom and a pilot (starter) hole in your base. I use black coated wood screws so they are almost invisible under the vehicle. Also, you can ensure your vehicle is firmly embedded in the groundwork by placing the vehicle on while the ground work is still damp and tighten the screw down to press the vehicle in securely. You can also adjust how tight it is once the vehicle is in place to make sure all the track and road wheels are touching the groundwork. Give it a try, you'll like it. Not sure if it would be a problem with judges, I don't build for shows, just my satisfaction.

Countersink a thin machine screw up through the base and into the underside of the tank. Twist the turret off and use a wingnut and washer. You can tighten a little bit to "settle" the vehicle so that it looks like its attached. Then reattach the turret.

The tie wrap idea is brilliant. Thanks.

Mike