i have made a hollow box thats unanchored so you can push it around, i removed the corners and added some thin line to make it look cool, whenever i weld it together so it stays together when unanchored, it all becomes weird and unconnected: (its supossed to connect and form a corner outline) 
Your picture is very confusing to me, but you might wanna try to union it so it all becomes one part. That should hopefully fix your issue, if I understand it correctly.
I can’t really tell what the image shows or is supposed to show. However, going off of what you’ve said in text form, I believe the resolution to your issue is to weld them together. Then, set all of their CanCollide to be set to false then using invisible parts you can create a custom hitbox for them.
This works on many occasions like when making secret doorways and such without needing to fix textures by using multiple parts, etc.
Try to make one invisible part at the center of the box and weld it to every box part. You probably just welded it wrong
Use WeldConstraints & & & & & &