wow what a great idea! i've been holed up in my room for the past few hours trying this out.
I have an idea for the sides so that you won't need the extra strip, so i'll try and explain without it sounding too confusing
1. fold in one strip on each long side of your woven mat
2. fold the bag so that there are 2 woven strips that will make the bottom of the bag, so it basically looks like a bag but the sides still aren't done.They're folded in but there are no sides if you follow
3. ok this is where it's hard to describe. On the inside of your bag, right up close to one edge of the bottom, pick up the 1 strip 'seam' that you've made, and fold a triangle from the centre of the bottom to the inside edge of the bag. When you do this, the side should stand up straight so you should see more clearly what you're doing
4. repeat from the centre fold to the other inside edge of the bag on the same side.
5.the two edges should meet perfectly along the side of the bag
6. repeat for the other side
does this make sense?! lol
thanks so much for the great idea!