i wouldnt so much as put it in a permission letter,
i would put together a package about yourself,
copies of license, and hunter ed certificates
letter should have a bio about yourself, such as why you hunt, your stance on safety, ethics, and respect for the landowners,
and give this to the landowner yourself, and ask him in person to look over the information and that you would like to hunt on his land,
leaving with a smile and a good handshake will do more to increase your chances than anything you could pen down on paper.