I recently bought an Antminer L3+ that was missing an endplate (fan holder part) so I decided to print one. This design has the fan holes where the hash boards have notches for the screw heads for the fan bolts. You can use M4 nuts and bolts or bolts and inserts.

I used nuts and bolts with the bolts coming from the inside out so the nuts were on the outside to hold the fan. This way I didn't have to deal with the length of the bolt intruding into a hash board.

You will want to print this with a heat resistant filament. I chose ABS and it's pretty strong.