Hi,
I have had a Megaslim for a couple of years, got it at Campmor.
It works. If you have a metal cage, it rattles around, but I have never had it fall out. It will keep coffee hot for a couple of hours.

I also have a travel mug that is made like a thermos. No handle, there is a little window that slides back and forth. I actually got it at a yard sale. Keep your eyes open, you might find something pretty cool just wandering around.

Target used to have a thermos that would go on a bike, they might still have it.