i like to use square top u-bolts in 5/16 or 3/8 diameter with 1/4"x1"wide straps [holes sized to the u-bolt diameter and strap length corresponding to include the size of the appropriate flat washer used].
sometimes though, square top u-bolts may be tough to find unless looking through a place like mcmaster-carr.
which ever way a guy clamps the springs, cut off excess bolt length and round the threaded ends. your arms and head will thank you !