My Theater Company moves several times in a season..sometimes into a nice theater and sometimes into an empty barn or open field.  This is what we did last summer:

The stage was made from about two dozen wooden pallets.  I scrounged them from the local stone mason's yard.  They are free, sturdy and can be stacked into all possible shapes and sizes.  The only disadvantage is their weight.  I had to borrow a truck to move each time and actually got a friend ( the same stone mason) with a boom truck to load us out of the last show and store the pallets in the barn.(Our backs had seized up)  The pallets are laid out and then covered with 1/2" x 4' x 8' fiberboard ( lighter and cheaper than plywd).  The fiberboard is screwed down to the pallets and holds them all together.  At the back of the "stage" we framed a 2 x 4 wall with the 2 x 4's at 36" on center. Between the 2 x 4's we stretched fabric and painted our backdrops.  The wall was screwed down into the stage and braced forward with knee braces.  On the front left and right sides of the stage we screwed 3' x 8' pieces of plywood and stretched two more backdrops over them.  This way we had a back wall to create a backstage and two "wings".  The entire thing could be built in about 4 hrs and broken down even quicker.  The only disadvantages were the weight of the pallets and the fact that we couldn't change backdrops.  You sound as if you want to have a curtain.  They are pretty tricky.  There is ( by nature) a lot of fabric involved and it is heavy and expensive.  If the reason for the curtain is to provide scene changes perhaps you could try rolling panels instead.  Think of a folding screen ( you know like the ones the starlets get changed behind) but unfold it and put some little cross feet and castors on the bottom.  These screens can be made cheaply ( just build big picture frames) and they can be covered with any type of cloth that suits the show.  I made several and we seem to use them constantly to create a room, split the action on the stage and sometimes the actors even get dressed behind them.

It is better to have a simple set with few moving parts than an elaborate "rickety" affair.  There is nothing worse than curtains that don't open, doors that get stuck, or lights that don't come on ( we suffered through that one last summer - fortunately during curtain call not act one).

If you invent a good portable proscenium and/or a way to change backdrops on a portable stage please share...I am struggling with the mechanics of that this winter.