As the “original poster” my (rather basic) questions after reading all the comments:
For my Lionel Santa Fe 2343, I would like to put the sheet aluminum as per @Lou1985.
To put the speaker in the fuel tank, I would have to grind or otherwise remove the battery lip with a Dremel. Any particular method and attachment recommended? Would a fiber cutoff wheel work?
As I measure it , the black baffle that came with the RS speaker still would not fit below my self installed aluminum plate. Is it correct that I would not need to use it, as the aluminum plate itself could act as a baffle?
What then would be the best way to secure the speaker in the tank over the now flush battery hole?
Out of curiosity how is it different working with a stamped steel frame vs a die cast frame?