Thanks for all your comments! On the grey bag I used the thickest iron-on interfacing I could find on the base and back, and the front, sides and flap I used a medium weight iron-on interfacing (it didn't iron on too well to the back of the curtain fabric so I tacked to on before sewing). I didn't use any interfacing on the lining as the material was quite heavy already. For the black bag I used the thickest interfacing on all the outside pieces and no interfacing on the lining again.
For the pattern, I just drew a sketch of what I wanted it to look like - added some measurements and started cutting with my ruler and scissors. It was easier to cut the black one b/c it had lines - but matching them up was a bit harder.