Gary55:
How'd the casserole turn out?
It wasn't sounding good. I'm not trusting Gary to faithfully follow the recipe... (I'm not Dennis) (nor his friend)
"Friends don't let friends bake while dilly..."
Actually Frank, you have a great idea going on there.
First of all the cross-sectional area and structural rigidity of the "T" section probably exceeds the tube.
Only problem is that it weighs several times as much, and the strength around the clamping is suspect.
Filling the tube is an excellent idea. I'd suggest fiberglass saturated with polyester resin as being strong enough, light and reasonably cheap for what it is doing. You could put some welding rods up the length of the tube to prevent catastrophic failure but I doubt it is necessary. One inch of fiberglass reinforced resin is plenty. Think "hammer handle".
I've used this method to beef up a thin metal hood in industry. It was double wall construction but flexed so badly you knew it would eventually crack and break. Plating it up was out of the question due to weigh concerns. We stuffed fiberglass fuzz and epoxy resin into the gap, packed in with no air bubbles. It stiffened up amazingly. 20 years after it is still holding up.
The spray foam is too flexible and compressible. You really need an compressible solid.