I tried out an old laptop sleeve, but I didn’t like the stress it placed on the middle joint or the way it took up space in my bag. I prefer to carry my keyboard “folded” — with the two halves stacked rather than connected.
I measured out the dimensions and got a hard case meant for a VR headset. https://smile.amazon.com/gp/product/B01N6N3J8D
The fit is near perfect. There’s a little extra vertical space, but the foam inserts take up that space as if they were designed for it. The small elastic strap holds everything in place. Per @cal’s suggestion about foam being abrasive, I’m looking to get some small pieces of velvet to wrap the foam inserts and ensure that they don’t wear down the finish over time.
It’s still not exactly a svelte solution. But I don’t think I could find a more compact way to transport it. I only carry my keyboard to and from the office at the beginning and end of the week, so this works really well for me.