there are a couple different ways. you could spawn an object float it to where you want it, then save and quit. you could also make a tower, and then delete all the objects on the bottom
not if the object is immovable like double boxes and fence boxes. anything that you can't move without forge.
http://www.forgehub.com/forum/forging-101/20-floating-objects.html Also check out the entire Forging 101 forum. It definitely helps. Has several topics regarding some basic and advanced forging techniques. Also, good luck with your maps.
Oh, I thought you wrote Sanbox I was gonna write like a Dbag . But try putting up a Double box and while you've selected it in Midair save your game then quit. THen repetadly do that until you make your platform. If you don't wan't your platform to be made out of Double box's (In game) just place objects ontop of the double box's and when done ,just delete the Double Boxes and the rest will still float.