Prusa MK3s upgrade fan duct

Update: 6 june 2019.
I've made a new version with even more clearance, and even more robust walls. It's visible in orange in the screen grabs (V1 in cyan, original in red). Keep in mind that this version is designed for use with socks, and because the "front" wall has been moved back 1mm, may blow onto the heatblock more.

A few tweaks to the Mk3s fan duct. Main differences are:
• The wall thickness and internal columns are beefed up for easier printing, and for increased strength.
• The gap between the fan and the heatblock has been increased to give room for a silicone sock

In the renders, red is stock, cyan is the new design.
Performance is pretty well identical to the original.