Looks like some people missed your "low cost" requirement. I'm going to get an old GI sling. Right now, I have an old strap that I pulled from a laptop case. It's worked fine...
As for aftermarket sling attachments, if you are going to leave it on, you should be fine with the sling attachments you have. If you are looking for something that is easy to pull on and off, you might consider some quick disconnects.