To thicken the stem a fan helps but so does placing the stem between your index finger and thumb and rolling it back and forth ever so slightly till the plant becomes bendy, this causes a stress on the stem and it will release enzymes to strengthen the stem because the plant will think it's not strong enough in it's conditions.
-DO NOT bend the stem so it breaks, or bend it too much that it creates a lean, you want to make it slightly more rubbery than rigid but don't bend too much.