Craig -

She feels plenty open - the pic might be misleading in its blurriness and perspective... The tubing for the strongback is 2" thickwall AL, the plate material is 1/4" thick.

Gonna seal the tubes somehow, haven't figgered that all the way out. Would like the caps removable, and also plan some tiny drain holes at bottom of the tubes...

All of this is aluminum, with the possible exception of the thru-bolts (and if I can use Al there, I will...). I want no galvanization if at all possible between the tabernacle and the mast. WRT strength of Al for this component, the tabernacle is built of hefty-enough tubing that it weighs about 7-8 pounds (scientifically tested by lifting it, then a 10# barbell to compare ).

Mast foot/shoe fitting - Still in the planning stage, likely to be built of composite to a tight fit. Gotta let the brain churn on it for a few days, now that I have tabernacle in-hand to see what exactly I am dealing with.