I have a flannel Wise Woman sling. I haven't had any problems with the rings slipping. I wouldn't use it if the temp was in the 100's, but depending if heat bothers you any sling may bother you at that temp.lol
It takes a couple of tries to get a wrap figured out but once you do it's not hard at all. I do a hip cross carry or a front cross carry and they both allow me to cover her right up. Once I put it on and then put Nadia in it she doesn't move much if at all.
For the hip cross carry I drap it over my right shoulder and bring both ends to my left hip and cross them. I then take the ends to my right hip and cross. Then I take the end in front and bring it around the left hip and tie in the back or side depending on where they'll meet. If I have more excess fabric I'll do an extra cross at the left hip and then bring the end in front across the right hip.
For the front cross carry, I'll start at my waist in the middle and bring the ends around to my back, cross them and bring them up over my shoulders to the front. I cross them again and bring them to my back and tie, or do another cross and bring them to the front and tie.
For both I put her legs in so 1 side of the cross is under each leg and then bring up some fabric from one of the other crosses behind her back and that is the section that I can pull over her head.
It probably makes more sense with pictures.lol