I resized the T-insert from the original project to fit imperial 3/4" aluminum tubing with 0.050" wall thickness (0.65" / 16.5mm inside diameter):
It's cheaper to buy this tubing from your local hardware store, I listed the amazon link only for reference. Wall thicknesses can vary quite a bit from different sources which will change the fit for this part, more than likely you will need to resize it some. Also remember that tubing is sized by the outside diameter, pipe is sized by the inside diameter.
I've printed and tested this part with PET-G, fits perfectly. Requires support at the square center is taller in height than the rods. Re-orient to print flat, not standing.