Not sure something else might work better, but I used this on mine. Make yourself a wicket out of 2 pvc elbows and 3 lengths. If you make it about 18 inches wide it would fit almost any dog. Make the uprights a little taller than your dog's withers. Doesn't really matter how tall this one is as it is not an official one, but a little taller if preferable. Then put it on the floor or ground and let the dog get near it with no fuss about it at all. If the dog won't approach it, a little peanut butter or easy cheese or any other treat that some scent or part of the treat would adhere to would help to get the dog to approach it. Do that until the dog is comfortable with it. Then lean it up against something, same routine.
Then you can try to measure the dog with it. The wicket DOES NOT have to ever go over the dog's head, you can bring it up from the back if you need to.
Don't know why that wouldn't also work for yours.
Good luck.