RepRapPro Mendel 40mm Fan Housing.

thingiverse

For enhanced cooling on your Mendel, especially for the first Multi-Head Mendel with horizontal X-Axis rods, you can use this design which won't work with the latest version featuring vertically stacked X-axis smooth rods. Attach it to an unused hot end location and secure it using the leveling screw; minor adjustments may be needed for clearance. Included is a solid-works model for customization or adding top layers to close it, depending on your printer's capabilities. Print at 100% once, then reprint the first two layers to join and close the structure. Additionally, gluing aluminum foil underneath can prevent heat-related drooping issues.