I've never used one. I store my saws in my house, and they ride in the back seat of my truck, if not the front so I've never needed one. I think if your saw sits out in the garage or bangs around in the back of your truck they could come in handy.
As far as suggestions the first thing that comes to my mind are do-it-yourself ideas. One is to take a Rubbermade Action Packer, or some similar plastic container, cut a slot in one end for your bar, and close the lid on the powerhead. They make different sizes, and they're pretty tough. And I've seen guys modify plastic milk crates to make racks for their saws. The cut a slot in one end for the bar, and the powerhead rests in the crate itself. They fasten the crate to their truck, run a bungee cord through the handle and there you have it. You could do something along those lines if you have to have a case and don't want to wait for one to get in from backorder.
Jeff