i just made 7 of these for $16. i used 3/8 bolts and some 3/8 i.d. aluminum tubing from the local fleet farm store. the tubing came in a 3 foot piece and i cut 3" pieces with my tubing cutter. the only thing i did different was i used two 5/16 washers on both ends of the spring. i'll give them to my friends.
They make great Christmas presents for your biker buddies! Cool man!
Wasn't the old rule of thumb just take the belt between your thumb and forefinger and you should just be able to make 1/4 turn any less to tight any more to loose.