@AKNscale I don’t have one of the sd35 but I do have the gp39-2 and it sounds very very good as is.
This post for the SD35 suggests the Atlas speaker/sound is pretty good in the sd35 too.
https://www.therailwire.net/forum/index.php?topic=45158.msg587225#msg587225Do you have an IM SD40-2? If so measure the speaker void in that, as I recall you can get ready made 3D printed enclosures that fit the SD40-2. JC mentioned it in his upgrading the speaker thread, which I can’t find the link for right now. If the same then that could save you making an enclosure. Just an idea.
I would go for the 73199 and you can wire up caps (same way as for the sd40-2) and there is pads for the aux3 and aux 4 for extra lights to use. So yes some hard wiring for the extras you want/need is involved but still a lot less than any other option.
Looking fwd to your next post on the completion.
Happy Holidays everyone