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Too bad that thing's not 36" tall. Being that it's only 24" & you want to do a horizontal, might be able to get by with a smaller element. I wonder if you do your doors like I have done mine in the past. I make my cuts where the hinges are going to go first. Weld the hinges, then complete the rest of the door cut. Seems to keep everything nice & straight without the hassle of squaring up the door with the body. Just my nickels worth.
Was you planning on the element being below the grates, that may be too close if it was horizontal, and would be very long. I think your best bet would be to make a vertical out of it.
the only thing i would be worried about is, if your heats on one end, will it heat evenly across the greats or will you need a gentle breeze/fan.
With the conventional horizontal wood burner, you can get even heat flow naturally, but you loose alot going straight up and not for cooking.
I think a horizontal electric is feasible, but will take some ingenuity to get desired results with out a lot of wasted heat/power.