RF design on air comp tank build.

Hi guys. Can I please check with more experienced builders a theory I have. My build is going to be a pair of 500litre air receiver tanks mounted on trolleys. ( 2 identical builds simultaneously). I have seen plenty of drawings and videos where a 6mm RF plate is welded in the tank just under the lower food grate. I understand the volume of air allowed for in the internal void under the RF plate should be about 30% of total volume. However, I am considering building a box under the tank and welding to the underneath along its length, and cutting in at each end to allow smoke/heat transfer. My theory is, I have to use a big piece of 6mm plate that length anyway, it just means I have to weld up the sides and ends, and that compensates for not having to weld with head stuck in a tank. The water pan would then be the curved bottom of the inside of tank. I then gain the extra volume in the tank to get an extra shelf in. Is there any reason why this would be a bad idea? Just concerns me I haven't seen anyone else do it, and if there is an obvious reason why.
Thanks guys.

Gareth
Hertford. U.K