UPDATE: The Internet thinks that buggies are on 120mm x 90mm bases, but I purchased a Deffkilla Wartrike, and the kit came with a 150mm x 90mm base.

A base I quickly mocked up based on measurements I found listed online. It's an oval base that should be the right size for the 8th Edition Ork buggies released in October, 2018. The oval measures 120mm x 90mm.