Last night I did a little experimenting. 3/4 inch copper pipe will
square up to fit over 1/2 inch square steel tubing. I just beveled the
end of a piece of 1/2 square tubing. Annealed the copper by heating it
dull red and then after it cooled used a dead blow mallet to square it
a bit as I pushed the steel tubing inside.
I only did a short piece, but it was pretty easy.

I agree rollers would be the ticket, but not sure it would be worth the
trouble for less than 100 feet of squared up pipe.

Dan