(I'm uploading this just because there seems to be very little people who know how to the Level Maker dupe. Credits to @Whytte for showcasing the glitch a bit more clear @MrRM1000 for discovering the glitch.)

First off, load in a save and pull up the transaction cancel menu. Leave this on until told to cancel. Now, proceed into the level maker. Upon going into the level maker, import or build a level of your choice. Then, pull up the menu. Select "Back to Hub" but don't select it again to finalize. Before pressing it you must remember that you have to go to the "Test at Spawn" option very quickly after you confirm the "Back to Hub". An easy way to do this is to use the hotkeys. Press space, click, s, space very quickly and if all goes well you should be teleported back to the hub. You can tell if you got it because you should see a slight glimpse of Beebo being teleported before he's in the hub. (You will also probably get softlocked. If so, leave the game.) After this, proceed into the Hecklands door. Now, press cancel on the transaction menu as soon as you start to enter the computer. Quickly run backwards into the lava during the cutscene without pressing E so you don't cancel. If done correctly, you should see Beebo in the sky above the Game Over Screen. Press E to cancel the cutscene and press the pause button. Now selects "Options" and select "Test at Spawn". Now you should see your level inside the hub.  (Note that some levels that are not elevated enough will be inside the ground.)