I've been considering getting a hippack (one of the R.E.load ones) for the same reasons, but I agree it's pretty wild to spend ~$40. I went over to the fabric store and picked up some burlap+velcro and made my own with about ~$1.00 worth of material. Reeeall easy, if you want to take a similar route.
I don't think the sweats gonna be an issue if it's hanging off your belt (read: not against skin). No waterproofing, but that shouldn't be an issue in the summer anyways. You can make a saddle attachment with a little velcro loop.
Looks like this:
And this:
