I have limited experience with round masts, but I found that the round titanium tubing I used was far more flexible that the square tubing (and also less damped) and I might suggest you make the seat angle attachment from the mast to the front keel a bit higher than with square tubing and also make it possible to adjust up and down a bit to allow for some frequency tuning. I made my 3rd gyro with a welded titanium round tube frame and would have liked to have been able to move that attachment for tuning. I believe optimal tuning will change for gyros as a result of rotor weights, rotorheads, and prerotators making total mass at the mast head different.