I have stiffened material to make roller blinds.
I mixed white PVA (woodworkers) glue with water - I used a bucket and made a 3 - 1 mix of Water:Glue. Put mix in bath and dip blind. Hang on washing line to dry.
If it were my blind I would spray all of it to get the same lack of stiffness all over.
The dipping process is not messy if done in a bath - the carrying to the washing line process is another matter.
Peccavi, October 2008