I just returned from my "workshop", and used one of the coathangers I have, from Ikea ( the one covered with green plastic, use that colour because that plastic will come off a lot easier than the other colours). You need to use almost all of the hanger, except the part that twists into the hooked part.
Just now I used one of the legs on a colapsible beach chair like this https://www.google.dk/search?q=collapsible+beach+chair&client=safari&hl=da&source=lnms&tbm=isch&sa=X&ei=_yAXUoCVL6OXiAL08ICoCA&ved=0CAkQ_AUoAA&biw=1024&bih=672#biv=i%7C4%3Bd%7CZbn52v10Wc852M%3A
It has to be around 15 -16mm i diameter, as the hanger will add the rest, ending up at around 20-21 mm. Wrap it tight, having the coils almost touch eachother, just do it by hand, it is not that hard. Drill a little hole in the tube you wrap the wire around, so as to keep it tight and in place. When finished stretch the spring a little to add enough distance for the coil to fill the entire lenght of the barrel.....ohhhh just thought..your build is a tube build...you need to meassure the inner diameter and then deduct 4-5 mm, that will be the diameter for the thing to wrap the wire around....make any sense?:-).. All the rest still applies to your build...:-)