Here is a hollow pole I made of 2 - 1x8's sandwiched around 2 - 2x 6's. The wood is larch which is very rot resistant. The pole is spliced together in 2 pieces with the rocket style house core splicing in as the 3rd section. Total length 21'........... 3' in the ground and 18' out of the ground. It was actually fairly easy to assemble in sections this way. This pole is very rigid. Another way this could be done would be to construct the bottom section out of treated lumber then use regular lumber the rest of the way up.
