BGE sells a two-tier cooking grid for the medium Egg. Alternatively, you can make one like you said, using bolts and nuts and washers (make sure you get stainless). Just measure the diameter of your existing cooking grid then head to Home Depot. I'm pretty sure they sell like a Weber replacement grid that is about the same size as the medium cooking grid, maybe a little smaller.

Good luck,

I have 2 mediums. For one, I took a grill from my old ECB, which fits perfectly, and added 4 stainless bolt/legs. The stainless hardware cost about half of what the whole ECB did.

For the other, I got the BGE grill extender.

Both work well, but have their own pluses and minuses. The BGE folds, and is easy to store and clean in the sink. The home-made's legs sit on the fire ring, and so there's more room for drip pans.