I once looked at those manila file folders thinking the same exact thing you are currently doing, as in making a larger deltoid glider, thinking that material was light. Well, another look and a scale told me otherwise.
I originally intended to try to get away with one ply but it is very flimsy indeed and when I looked at the overall weight of TWO plies....it is heavier than ReadiBoard!! Plus, I still did not like its overall feel. Yea, being able to use that folded edge seemed good but the scale doesnt lie.
You also mentioned about possibly just mounting a forward motor tube and ejecting the motor, ala Astron Falcon style. Since your glider seems kind of large for that, coupled with the two plied heavier stuff, I would think that you will not get enough of a CG shift for boost with an 18mm motor, especially once the propellant burns out and it coasts. Obviously, the farther forward you can mount the fixed pod the better, but then you start looking at spar flexion as well as increasing the odds that longer body spar will break if it glides into something or even warps a bit over time (a definite no no).
Besides the delta you are already showing, what other surfaces are you looking at here? And what overall size? Darn, if I had access to a camera I would post several dozen shots of what Im sitting on that you guys dont see or hear about.
As light as Readiboard is you might be able to make a good popmotor job nice and easy as long as you size the main delta wing right. Obviously, I dont recommend this for standard convies since it usually has warpage inherently in it. Still, theres this 40" span Transwing type Im fixing to slap together.....