If you want another one-shouldered sling, I'd say get a hot sling with the padded leg rail. It makes a great hip carrier for a toddler and a great sling for a newborn. And the price is right, plus it meets your need for coolness.
If you'd like a 2-shouldered carrier, I'd for for an ABC like a Kozy (I think she has some options under $50), an EBMT, Baby Backtie, etc.
I'd second the Moby rec except for the hot factor. Living here in the hot-muggy-hot deep south, it's just too many layers of fabric for summer.