I’ve recently gone to several hardware/construction stores in search for suiting MDF plates for my MPCNC base. In Norway they usually come in 1.2x2.4 m size. If you want a decent thickness (around 15-25mm I would assume), they cost a lot and weigh a lot! Not many stores have the option to cut the huge plates on site either, other than with a hand saw. I chatted with one of the store clerks, and he suggested that I used “packaging plates” (emballasjeplater). They seem to be of some kind of MDF, and are about 1.2x1 m I would estimate. He actually gave me two for free, he had lot’s of them lying around to use for in-store packaging. I glued them together, and the base seems very stable and more than flat enough. So - if you want to keep your costs down, or just want to escape the hassle with the big plates, ask around for free plates, they have them in most building materials shops!