Four ideas:
One: Extend the antenna in the Hawkeye so it can be placed outside of the mailbox. Inside the Hawkeye there is a small loop of wire. I don’t know if this can be extended by cutting the loop and placing a wire in series or if just attaching a wire to it would work.
Two: Use the sensor for the garage door detector. The mailman would wonder what the heck that big box on the door is but it may have better range.
Three: Buy a plastic mailbox.
Four: Get one of those vacuum delivery systems like the banks have. This is probably more expensive than the RF extender you spoke of but you wouldn’t have to check it at all than.

Chris