This is an attachment for the diamond hotend. Print it to use it with your Rostock.
This is specifically designed to work with the rostock Jaws and joints, without the need of modification to any of the existing parts of the rostock.
The design is such that the end effector does not need to be printed again. The attachment sits on top of it and can be screwed from the bottom.
If the tower holes are small for M3 Bolts then i would suggest to use M2x15 bolt on each tower.
Put the bolt head on top of the opening, hold it with a nose pliers, heat the bolt with a soldering iron and push it inside the tower. Then you can put nuts and washers.