If your looking for some prepared text, most likely you won't find it. A medicine wheel, as you know, is a native American construct. Some of them date back 4,500 years. Stuff isn't written down unless some white man made an attempt to do so (and possibly embellish) and then that would have only happened in recent years.
As it has been explained to me, the wheel is symbolic of mother earth womb.
It's open to the west to allow entrance and exit (like the birth canal) (west is the direction of where the spirits reside)
When we wish to connect with mother earth and to share knowledge, we return to the womb where we can grow spiritually. When we exit the wheel with new knowledge, we are reborn to the world of the people.
It's absolutely appropriate to open the wheel with a prayer. You might ask for guidance, wisdom and understanding.
When leaving, it's appropriate to give thanks.
Also appropriate to use sweet grass as a smudge, cleansing self and site, and filling yourself with the spirit with whom you wish to connect.
With that in mind and your intentions going in, speak your heart when opening and closing. I'm sure that is more than sufficiant.