The only way to make structures invisible is to use map-specific locations, like in Erosion's Grifball area ceiling or the top of the space base on Impact. Large objects merged into certain spots of the maps geometry will become invisible.

: Also, why can't I get my Extraction Target structures to leave when the timer
: finishes? Or are we in deed limited to only use the Extraction Target
: items in the objects menu? If so then that just sucks.

Like, a 1x1 block? Doesn't work. They will only vanish if the Extraction site times out, not if it is captured.