Recently in Forge, I have been entertaining myself by performing various self-splatters. Whilst splattering myself with the double box, I noticed that wherever the box makes impact with me is where the blue tear-drop icon appears, yet the double box keeps moving. I believe that this is perhaps the ONLY way to have a box spawn point in one location but the physical box in another. Furthering my discovery, I realized that I could then interlock with the double box that was not there at the moment. Obviously, this method is not preferable on account of the fact that you don't have many options for placing the objects. However, if you plan on having a series of double boxes at a set height and not too complex pattern, this just might work. I might put some screenshots up but I think I explained it enough.
There is another way you can do that, but you have to have another person in the game with you. The person who is not the host can do it. All you have to do is press A on an item in the item menu followed by X straight afterwards. You should have the item menu up as well as holding an item. Then you just spawn a different item normally, then delete it. Start a new round and the item should be floating.