This is a simple fan duct to keep the power supply fan from blowing on the heatbed. Redirecting the airflow helps the heatbed warm up much faster and use less power. It also helps a bit with the noise.

Source is here: https://cad.onshape.com/documents/139ab3523170cbc9573979ef/w/983f4d6cad7a710ed5ba1576/e/2120583754034f15fdef2cac?renderMode=0&uiState=679070115fd8922d0be803be

Inspired by: https://www.thingiverse.com/thing:3321450