if I do need to turn a tube I have a set of different sized saftey pins in my sewing machine box. I use the biggest that will fit intot he channel. However for bag straps I nearly always use regular waistband slotted interfacing which irons on. Then you fold in the edges on the folds and top stitch. Gives you uniform sized straps filled with lovely stiff interfacing. works a treat!
It comes in 1" and just over 1" normally. I noticed recently, Nancy Z has something similar out but it is fairly pricey compared to normal dressmaking waistband interfacing. However she does have some narrower versions.
Becks