If you move the rod that holds the natural shed dowm a bit, the shed you pull will probably improve. It will decrease the natural shed a bit, but that one seems big enough anyway.
When I weave a Moebius my first heddle rod has heddles of at least 15 cm, the second one has shorter heddles.
By the way, one of my looms used to have a 2 cm shed, but since I always inserted a smal weaving sword first and placed that on its side, it was not that much of a problem.
I'm quite happy with the size of both sheds at the moment - the artificial one is just large enough to not be a bother, and not to need anything extra stuck in to open it some more (which is something I'd rather avoid for the purposes of this demo loom, since it might confuse people into believing the large ones had huge paddles to open the sheds...)