Ah, problem is the fan were designed to mount to a metal plate, not a wood panel.
Couple of easy suggestions (test in scrap wood first to see what you think).
Assuming these are 80mm or 92 mm fans. . .
You could cut a larger square hole in the wood, cut a metal scab patch to fit over the hole, cut a circular hole in the metal patch, bolt the fan to the metal and the metal to the wood, but that would look ugly.
Option 2 - if the fan is about 3/4 inch deep, you could use this approach on the INSIDE edge of the side wood.
Option 3, cut a square hole for the fan, put the next larger size fan grille over it, 92 or 120 mm. Use zip ties or some combination of screws/washers to hold the fan to the grille.
Option 4 - cut a square hole the size of the fan, in the side wood and use contact cement, RTV, or some other goop to hold the fan in place.