"Good idea, but leaks with the 2L and the 3L hoser bags"
I usually try to carry my 3L platypus bag attached to the hose and another 2 or 3L bag while on certain hikes. Although these caps are a good idea for securely closing the additional 2 or 3L bag, it leaks. I was hiking up the Climber's Trail at Timberline Lodge (Mt. Hood, OR) a few months ago and, when opening my bag to switch out the reservoir bags, found the inside of my bag wet with a small puddle forming. This wasn't from condensation, but slow leakage from around the circumference of this cap.
Prior to buying these I had used other bottle caps as replacements to my platypus bladder, however although they are a similar size they did leak slightly which these caps do not. For the cost these are an excellent way to save from having all of your gear soaked from a leaky cap!