When you practice, you need the right equipment.
The average length of a casket is 84 inches; width 28 inches, and height 23 inches. We should also point out the difference between a casket and a coffin (believe it or not, they are two different things!)
A coffin is the six-sided box often used in vampire movies—it’s wider at the shoulders and tapers down towards the feet. Coffins are still used occasionally, but they’re more of a specialty item now. A casket is the standard four-sided box most often used for modern burials. Casket dimensions from here.
Here are some of the things you will find very helpful:
A practice hearse (calling it a “coach,” is easier to use around the next-of-kin)
Here are the rollers for what we use at Patrick Air Force Base
A mockup. This is the device that is placed over the grave.
And, don’t forget the interment flag, you can get one here: US Flag Store.