Hope this isn't just stating the obvious, but the maker sells a .45" pole with the end tip. Have you contacted the maker about a solution for the larger diameter pole?
If that doesn't work, please PM me with the inner as well as outer diameter of the .600 pole, and I will rummage around through my boxes of accumulated tubing and parts to try to come up with something. There must be some kind of an end cap on what you have already. At least 50% of the time being an incurable packrat has its benefits.