That shouldn't be a problem. I started my SFG on existing grass. I covered two beds with a weed barrier and the other beds with newspaper to suffocate the grass and weeds. I added my soil right on top of both. They both worked equally well. Since you are already starting with bare soil I would skip installing anything on the bottom. Just add your soil and plant!

Here is a link that might be useful: Kim's Flower and Vegetable Garden

Thanks.
I was kinda wondering if root crops would transition from Mel's mix to regular soil on the way down. I would think so, but I haven't seen/heard any mention of this.

I think they would as long as the soil below wasn't as hard as a rock. I grow Scarlet Nantes carrots in my SFG bed on top of heavy clay soil, and they seem to be getting long enough to reach the soil under the box. The only way you'll know for sure is to give it a try. I bet it will all work out beautifully.

Thanks again I will give it a try.Hopefully this will be one advantage of having sand for soil.