If you want to be really minimalist about it, I'd try this experiment:



No wait, if it turns out this is enough to prevent the chain being thrown (probably in conjunction with a SS ring), then you can disassemble the RD's pivots and drill new holes for the springs, to set a higher chain tension. Harder than usual to reassemble.

BB derailer pulleys, as always, are a good idea too (plastic not ally).