(Please be gentle, I’m not good at blender.)
I’m sure its my fault, but why does do I lose this detail when baking?
I’m honestly not too sure why this is happening. I’ve tinkered with the “selected to active” stuff a bit, but no matter what the two long “cutouts” on the top and bottom of the sides of the shipping container just don’t get baked?
Welcome 
…
i’m not sure what you mean by cutouts… what i see is that mostly (even on other images than the next one)
at the top there are mostly just one long thingy… but that might no be a real problem.
Also think about it… what does this one image show us ?? The highpoly ? Well… this looks fine… Problems with the baked normal map ?? What map? Your aren’t showning one nor the usd normal map on the lowpoly…
Anyway:
The reason for this could be a l ot of things… for once using some rectangualar geoemtry so that the normal is not any incline… or you may have to use a cage and then also adjust the size…
So i guess:
You may have to elaborate this to get a more suited answer.
Yeah that’s my bad, I’m apparently limited in how many pictures I can embed. But I think I’ve gotten if fixed now. My apologies. 
Have you tried setting the “max ray distance” a bit higher in the selected to active settings?
Ps if you stay logged in, read a few posts and give a couple of likes you should be promoted soon to be able to post more images.
No sweat… but you may just tell… i’ts a community… everyone can learn something… ( i do all the time ) 
