I didn't read threw all the posts of the thread so I might be repeating something. Depending on how much money you want to spend, you could use something cheap and light such as a base (Styrofoam) in which you can transport easily and then reinforce it with quick drying concrete. Stack styrofoam to build what fortifications you want (small fighting positions, small bunkers, or even full sized buildings if desired) then add some water to the quikrete and cover the styrofoam to reinforce it.
Dwayne and company used this method for some buildings they made at D-Day adventure park. Brilliant. Now, for what you need, you don't have to go all out with buildings and stuff, but it would be easy to make some small dug in fighting positions reinforced with a 1 1/2 ft. high styrofoam/concrete wall in front of it. You could make some pretty nifty fighting positions for a relatively limited amount of money..... that will LAST!!!!
Might be too expensive or elaborate for what you are looking for, but it's just another idea if you want to use it.