This is a 2020 frame brace designed to stiffen the frame on a Ender 5 Plus. 

The honeycomb design was to match the awesome Ender5 Plus skirt/enclosure design here: https://www.printables.com/model/557066-ender-5-plus-skirting-and-lower-enclosure-base-cas

120mm x 120mm x 20mm.

Holes for M5x100 screws