This is a really bad hack of someone else's STL for a single mosfet holder from the Anet A8. The Hesine M505 is a close cousin to the A8 but uses the Melzi 2.0 board rather than the Anet's custom driver board. This mod lets you attach a single mosfet (for the bed heater) above the Melzi board by replacing the standoffs with this part.
So, like I said, this a really shitty hack since only the STL file of the original part was available. The model is bent up and requires support to print. Actually the support material has to be left attached for it to sit flush on the side of the printer. I actually still use this part, I'm only including it here for my M505 brethren who are left without good mosfet mounting options. Actually they are still left without good options because this one sucks. However, it works, so it has that going for it.