In my experience you can still just sew it in. I just bought some heavy weight interfacing that is iron on for use in a wallet and after repeatedly ironing it to my fabric it didn't all fuse to the fabric....So i just sewed as if it was fused to the fabric completely. If the interfacing is cut the same size as the fabric and you pin well while sewing you shouldn't have a problem with the interfacing shifting.
If you got the thin iron interfacing this probably won't work because it won't give the bag much structure without ironing it on.
Hope that helps!