I was having the same problem. Before I had a chance to go out and buy a pizza stone or whatever I tried this: I wasn't able to use my old cookie sheets for baking in this oven anyway, just a tad too big. I had to buy smaller ones. But I noticed I was able to insert the old cookie sheet onto the bottom rack of the oven. How can I say this? hmmmm ... okay, instead of putting the oven rack on the lowest setting in the oven, slide the cookie sheet in its place instead. And just leave it there permanently. It's a bit of a tight fit, but that's good, it won't move. It's taken care of the burning-on-the-bottom problem quite nicely for me. And I got to recycle my old cookie sheet. Of course, not every cookie sheet out there will be the exact fit as mine was, just so happens it worked for the size I had.
You understand, of course, that I'm not saying you cook anything ON the cookie sheet, it's just taking the place of the pizza stone or clay thing to disperse the heat, for now.
And your oven rack sits in the middle or top. Clear as mud?! And, no, I won't post a picture - oven's a disgrace