How do you guys join things like this? Usually, I apply two different materials, one to each element, and attach them. Then, I weld, until things start joining, but it’s messy and joins things I might not want to be joined. Once joined, I select by material ID and detach. Like I said, it works, but it is sloppy. Is there are a better way?
and target weld is painfully slow.
I bought this years ago, but I don’t think it worked for me. No demo and it doesn’t say if it works in MAX 2017
Can you delete bits of mesh from the larger outer piece of geometry so there’s a gap between inner/outer pieces. Then join them as a single poly and select the 2 edge loops and bridge them together?
In compound objects there’s shapemerge. Use that in cookie cutter mode (it has an invert feature if you need it). Then stick edit poly on top go into subobject, it will have the relevant edges selected from the “cookie”. Soft selecttion then adjust as needed.
I’d select the two borders, create a spline from them, combine them and apply a edit mesh on top. Boom cap hole 
simply first attach them the select the border and cap the hole,after which you can add a subdivision modifier to help with the geometry look more sculpted
Depending where the camera needs to go, and how close it gets to the seam, I sometimes find that simply poking one surface through the other can work just fine.