Conceptually it could work. Yet the time and effort to get it right would be immeasurable until after you were done. These gravity feed units are simple in concept, but expensive in parts and labor.
With that said from my experience, which is limited when compared to some of the guys on here, I would never try and make an air tight door from a rounded metal surface. Just getting the door of my reverse flow to sit and look right was a bit of a task. Also insulated cc and fB is a must on a gravity feed. Controlling the conversion of solid fuel to heat means you need to have the retention of as much energy as possible to precisely control temperature.
So my suggestion would be if your married to the 30 gallon tank think about making him a pellet popper. My friend just built one out of an old Vulcan gas stove and it works as good or better than anything you can buy.
I'm sure others will chime in, but that's my 2 cents.
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