They are called 'ghost pieces'
you need to use the FCML editor if you want to make them. you can find it under: tools http://fc.sk89q.com/
click at: insert -> level objects -> Dynamic rectangle
it should say: DynamicRect (0, 0), (50, 50), 0 in the text box. you need to change one of the 50's into a 0. you need to decide which one because one will be diagonal and one vertical. you just need to click at upload if you're done, you will get a level with the name: Test.
You only can rotate the ghost pieces so you need to get the right length