First, to start out, I will say that some of these ideas are not completely original or mine. This is just a compilation and a general consensus of ideas I've seen plus some of my input or maybe an original thought or two.
Obviously everyone agrees that the goop build should be able to be saved but the question is how to do so without the potential for abuse.(like lag, duping, exploiting, etc.) One way to do this is a separate stash that allows you to put the whole goop build in a large box, and when you press a button, it sends it inside of the separate stash to be stored. There is a small issue of people being confused how to use the machine and people getting inside the machine but those aren't real issues. Just have them be teleported to the/a stash dimension. The the only real issue is people putting too many things in the machine and causing lag when it's taken out. But this is solved by just a simple item cap and possibly a separate, higher, goop cap. You would take builds out by just simply selecting it on the screen and pressing a button and it would close a door, and the build would appear when it opens.
Another idea is a separate section in the stash that allows for the saving of builds by just putting it in the stash like a normal item, except it appears in a different section of the menu that works differently. May have an issue with putting large builds in if people don't know about taking the build from your hand in the stash. May have a higher chance of glitches, so I think it might be easier to implement at first, but taking care of the bugs may be a huge hassle and lead to tons of duping glitches if you're not careful. Basically you would do this by just storing the connected items under the "parent" item as if it were a backpack except you would only be able to see what it looks like/what is in it and not be able to move it.
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