I'd use no more than 10% acetone to thin the epoxy. It doesn't take very much, even 5% makes it brushable. Al probably gets away with it because Dallas weather evaporates the solvent much more quickly than Chicago weather does. Entrapped solvent is bad news for any finishing system. Slow cure and poor hardness can be the obvious results.
Epoxied 0.5oz cloth will never seem very rigid by itself, its just too thin. Even fully cured it will flop around like any other thin plastic sheet with ordinary epoxy. Its when it is attached to something else that you get rigidity.
Another thing to note is that with non-soluble resins like epoxy, you have to get enough on to fill the cloth first time around. If the fibres aren't filled, adding epoxy after the first lot has cured isn't going to add strength, just weight. You can do it with dope, because each subsequent coat melts into the existing coats, but more epoxy just sits on top of the hardened epoxy. If there are voids and unsaturated fibres, they'll stay that way.
I say try again with 10% acetone, make sure there is enough or slightly too much resin in the glass, hit it with the toilet paper and then maybe heatgun to help the acetone evaporate before it gets trapped.