I used a 1/4" mandrel in a power drill to make my springs. I couldn't use aluminum because wouldn't hold up under the beating it was going to get.
Just an FYI... I made my shirt like in your picture and got a ton of ribbing from the guys (and several girls for that matter) at the next SCA fighter's practice. Everyone kept telling me I had the pattern turned 90 degrees. So I checked some historic photos in Stone's Glossary and spent the next week fixing it.
I pretty much beat up my hands making mail and doing armor smithing. I have RMS (repetative motion syndrom) in my right wrist. My hand goes numb if I grip too hard or too long. If I do it again I'll get my rings pre-made from a place like this www.worthco.com they make split rings and jump rings out of stainless steel for fishing tackle.