Read this: How to Geomerge You can either use the first method or you can interlock items to take the place of doors. There is another method specificly (forgive my spelling) for Sandbox where you take an object and swirl it around with the right trigger and right stick while it's touching the ground, then you will have to try really hard to get it where you want it and not get stuck outside of the map, then save & quit.
well geomerging in the crypt is completely different then geomerging in the sand. For the sand what i do is set the object as close to the sand as you can without it actually being on the sand. Set up teleporters around and on top of your object to act as guides. grab the object from behind and just push it into the sand. the teleporters on top keep it straight so it goes in the way you want it. For me thats the most accurate way. I havent done much work in the crypt so i cant really help you with that one
I used to ask this question alot. but i found out that the variety of objects dont need geomerging. There are many diffrent sized boxes so u dont have to geomerge. but as for the geomerging in the walls, ur stuck