I would believe it would be identical to a Union marker. See:

Tips on Placing a Flag on a Veteran's Grave

For grave markers, purchase a flag with a 3/8 inch staff for a tight and steady fit. Place the flag holder and the flag in the center of the head portion of the upright-style of monument.

https://classroom.synonym.com/tips-on-placing-a-flag-on-a-veterans-grave-12084114.html