don't know what kind of a vehicle you have, but i built my own as well. i have a rhd jeep, and took out the left front seat and put in a flat carpeted floor as well. works great not having the seat in the way when grabbing a bundle of mail or parcel from the back. mine is high enough to put mail under it, and shift the transmission and operate the parking brake, but low enough to be able to see the left mirror even with a bundle of mail on the tray. any handyman should be able to make one for you reasonably cheap. start by making a cardboard pattern, and figure out what you need to hold it up.
