Power-Up Geomerging
I remember some FH staff members announcing that there were no door like objects found in Sandbox, and that we'd need to go back to the "old-school" technique. Well I think I've found an alternative for doors.
Note: I do not own the Mythic Map Pack, so I do not know how things will turn out on Sandbox. I do not know how far this technique can get you, as in, how much you can do this before the object goes all crazy. And I have not tried angled merging with this technique, or tried it with any other object besides shown.
Step-by-step Tutorial:

1. Place the object you want to geomerge and brace it.

2. Place the object to spawn at No, and do whatever respawn rate serves you best. I had used some spawn points to point out the corners of the double box. Start a new round, place a dumpster at each corner of the double box, and place a power-up on top of it. And delete the dumpster. The dumpster is to set a uniform height for the power-up.

3. Force spawn the double box, and it should look like this. See the custom power-up sticking out at the corners of the box.

4. Optional: Put some braces on top, best if you do if you want the best quality of merge.

5. Double tap A on the box
OR grab the box and save and quit.

6. Delete the braces, and notice the box sank down accordingly to how much the power-up was held within it, just like the way foundry doors do it. (Don't bother telling me that this isn't perfectly parallel with the ground, I know it isn't, this is most likely because I didn't save and quit while grabbing the box.)

7. And you're done!
Please leave your thoughts and comments.