Snap-in 3030 fan mount for 1515 extrusions

Made as an alternative to the stock 3030 fan mount. This one is just one printed part and needs only half the screws and heatset inserts.

Props to the ZeroPanels design for creating the snap-in geometry and to Blamm for giving me the idea to borrow that geometry.

I borrowed the snap-in geometry from the ZeroPanels and made the rest of the model myself.