And one very good source to get information about medieval letters is the Paston letters, a collection of letters written to and from members of the English Paston family between 1422 and 1509. These letters are available in full view on googlebooks.
A lot of the letters are too long to adapt, but the greetings and closings are neat --
"Right worshipful brother, I recommend me unto you, letting you to weet that my Lord Stafford was made Earl of Devonshire upon Sunday...Written at Windsor on Monday after Whitsunday in haste, etc."
"Right reverend sir, I recommend me to you. Please it you to know this same day came to me the sheriff of Norfolk himself...By your well-willer,"
"I greet you well, and send you God's blessing and mine; letting you weet that on Thursday last was...Norwich, before Michaelmas."
"To Master Writtill. Right worshipful Sir, I recommend me to you, thanking you of your great labour which I have not as yet but I shall deserve to my power; and furthermore like you to weet that I have thought right long after you...Written at London, the 10th day of September. By your friend,"
Here are some texts that I've constructed on the basis of the Paston letters and similar:
To owre dear cousin the countess Judith de Northumbria. Right worthy and worshepefull ladye, I recomaunde me to yow and thank yow for þe good, trew, and diligent labour ye have hadde for the mattier of the quenes of Drachenwald. For yowr conseil and thee manhede yow have shewn to us and otheres, we wish and firmly command þat ye be everilkquar accompted, nombred, accepted, and receyued into our Order of Corteisie. My wille is that ye shew þe chief signes of this honour, ane whight edelweiss wyth a centre of redde.
Written at Knightes Crossing þe x day of Iuyl, a.s. xlv. Yowre frend and lege ladye, þe quene of Drachenwald,
To our right trusty and heartily well-beloved Guntram von Wolkenstein, Master of the Laurel, we greet you well, thanking you as heartily as we can for ourself and especially for that ye do so much for our house and court, which I trust verily ye do rather for the great love that ye deem I have thereto. And forasmuch as we purpose to thank you for your rightful favour showed to us in all matters that toucheth us, we pray that you proudly and humbly bear our sigil, and that in so doing you be long preserved in honour and prosperity. Written at Adamestor, on the 6th day of November, a.s. 45, by your queen
By the king ryght trusty and welbeloued. We grete yow ofte tymes wel. ffor asmuche as our right dere servant count Cadogan map Cado hath long tyme in certain matiers þat greatly concernen þe gode weele of us our Landes lordships and subgittes yeuen us gode counseil and reid, wherefor we have yeuen our assent roial that he may bere þe seal of þe kyng. Yeuen vnder our sigil at oure town of Aventiure þe iii day of January, a.s. xliii.
And this is only a taste of the possibilities! So next time you're working on a scroll for one of these awards, and need some ideas for a text, troll through the Paston letters or the like and see what you can find!