You can measure the length of your rudder tube and add a bit.
If you can't dig a hole I would suggest biting the bullet and raising the boat on a travel lift for the little time it takes to remove the rudder. Raising the boat sounds risky to me. The weight of the boat should be on the keel. There are lots of members here who have expertise about this - I am not one. Maybe someone else will respond. Unfortunately, it's decent weather in the East and a lot of us are trying to get our boats in.