Backdrops (Custom Backdrops) <drumroll>OK, the magic formula for making backdrops from scratch (like the Dark Star backdrop):</drumroll> 1. Build the model (use a size similar to that of the stock backdrops), using the MTM1 transparent texture. 2. Select all faces 3. Goto "Groups>Set group magic number" and set it to -2147483648 (Strange number? No way! It's the smallest possible signed number that can be stored in 4 bytes) 4. Save as Backdrop (this will not recalculate the magic numbers, thus preserving the -2147483648).
Note : When adding faces to solve the hall of mirrors, one should also set the Magic Number of the new faces to -2147483648.
If you need a more thorough explanation of bins and their file format and how they're used in BINedit, please go here. - Thank you Oliver Pieper.