There are many PET materials available in 0.125mm and 0.175mm thicknesses which either have surface coatings or surface treatments to aid the screen printing of inks.
The company I work for uses a lot of this material which we buy in 2' x 3' sheets.
For our purposes, there are two types of coating/treatment. One is suitable for solvent based inks and the other for water based UV cured inks. It is the latter type which I think would be more suitable for emulsion.
A while ago, I collected together some samples for Denise to try out but to my shame, I have just realised that they are probably still in my drawer at work. If you're reading this Denise, please accept my apologies. I will try to post the pieces soon.
There are also some polycarbonates and polyester/polycarbonate blends which might be suitable. I will go through what we have at work tomorrow and post other materials/sources.
Note: Mylar is a DuPont trade name, not a general term for polyester.