There shouldn't be a hole there. There is no need for the heated air to be mixed.
On a side note, that looks like the original furnace that was installed when the house was new (probably 40 year old Airco, which are incredible energy-inefficient, probably 50% efficiency). It would be a good idea to have an appliance tech to check and make sure the heat exchanger isn't damaged and leaking carbon monoxide.
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