craftgrrl | Making Stiff Knit Fabric

I have plans to make a bag for my friend for her birthday. I wanted to make it like a small messenger bag. So for this I was wondering, what is the best way (besides felting) to make the knit fabric stiff? So far I've gathered that if I knit it with smaller needles than the gauge says, it will make a stiffer fabric, and if I turn garter stitch sideways it shouldn't stretch that much. Do you find this to be true? I am using Bernat Softee Chunky yarn which has the gauge of size 10 needles, so will 8 be enough to make a stiffer fabric, or must it be smaller?

Thank you for your help!

P.S. I apologize if this was addressed previously, I did not find it in the memories.