This is a socle and a fan duct to fit a regular ATX Power supply in a Mac Classic case.

The socle is use to elevate the power supply, so that a regulatr CPU+fan can fit below (some restriction apply).

The fanduct match the rear opening, so the fan from the PSU can be used as an exhaust fan.

For fixing the PSU and the fan duct to the support, you will need some  #6-32 screws (a few millimeters longer than those provided in general).

For fixing the support to the iron chassis, you will need some M3 screws and washers.

My notes on the complete project can be found here