Freezing Burgers - without sticking

I'm trying to plan ahead and freeze burgers in a batch. But they always stick unless I freeze them flat on a tray with no layering, and only then can I carefully lift them off greaseproof paper to put in a freezer bag.

I've already tried to lay greaseproof paper or cling film over one layer, and then make another layer. They just stick, especially to the cling film - also tried foil - same issue.

Any bright ideas around this? Thanks