Skysteelhead -
Your tube sounds a little large to me. Are you looking for a single or two man operation? One of the challenges in using a tube for digging geoducks is getting the tube in and out of the ground. The other factor you need to decide is whether you want to get your body inside the tube as you attempt to reach the clam our just use your arm?

If you are going to rely on reaching inside the tube with your arm anything over 30 inches long or 18 inches in diameter is wasted and makes the task of inserting and extracting the tube more difficult. If you are going to get inside then the tube needs to be large enough to allow your shoulder to get inside.

35 years ago my father and I made a couple of tubes out of 30 inch pieces of 18 inch corrugated culvert. An inch below one edge we drilled a couple holes that allow for us to insert a piece of re-bar that serves as a temporary handle. With the handled insert can screw the tube in the ground 3 or 4 inches, remove the inside material repeating until able to reach the clam. Once the clam is removed it is a simple matter to "unscrew " the tube. My son and I are still using the same tubes and my back really appreciates not having to pull the tube out of the much with brute force.

Tight lines
Curt