I created this velocity stack based on pictures I found online. It mates with a K&N air filter (model RU-4410 or RU-2990)

There are left and right variations of the velocity stacks. I also added a flange at the end of the stack to the neck of the filter sit perfectly. 

Back of the stacks are hollowed to save filament material

I printed mine in Polycarbonate (PC) which should withstand the gasoline and engine temperatures. 

Bolts are based on the stock air filter bolts.

Printed at:

 0.20mm with supports for the stacks

 0.16mm without supports (with brim) for the bolts

Hope this helps someone!