I would suggest tilting the back of the harp up and making sure it’s deep in your mouth. The natural curve of your lower lip when the harps in there will create a channel.
It’s a more relaxed way of lip purse playing and doesn’t result in such a tired mouth…. And it gives good tone.
Using your tongue isn’t necessarily cheating, in fact it is the basis of the often lauded ‘tongue block’ method, but it depends on how it’s done.
If you are using your to urge to block notes on one side and play one on the other, then that’s legit tongue blocking. If you are curling it to blow a single note down the middle, then that probably isn’t the best to work with.
Whatever you do, just enjoy yourself!