how about an artifact that combines two structures into one, making it stronger and smaller than both combined
for instance 2 nanite labs are +3 research for 9 space each or +6 for 18....use the artifact and let's say it becomes +6 for 12 space..... call the artifact a structure merger and the structure itself a merged laboratory, or merged refinery<for mining>, or a merged trade hub<for artifact>, with whatever + being the level of bonus gained
in the above example it would be a merged lab + 6
you can also make it so that you can only combine structures of the same bonus, limiting the number of them on a planet
mission reward or elite drop ?

add this to the fabrication plant drop or the refinery center drop.

nope.... that'll make a +6 way too cheap and too small... it'd cost less credits and space to do it that way than it would just to put a +6 unit on the planet...

if this is a very rare or mission limited artifact then OK, or if it's more common but costs a lot of CR to use or something then okay that would work too.