Here is a trivet I made from an old moto brake disc. The only work I had to do was to make little legs for it. I thought about using bolts but i couldn't find as flats ones in order not to exceed the height of the disc.
So I decided to create ones using threaded rods. I welded washers on top of it, then I buffed the excess of welding and I finally used a grinder and a drill to reduce the diameter so as it fits in the fixing holes of the disc. Then I just had to fix it with nuts (two ones to have a good height for a trivet). For a better finish, I sanded the disc.
And here is a very low cost trivet for moto lovers!