Materials like Plywood, Melamime and Wood Panels come in similarly sized panels based on feet. So ply comes in 2400 x 1200mm and from Bunnings where you can get cut down sheets, there are sizes like 900 x 600, 1200 x 900, etc. So keep in mind when you are designing your enclosures that you keep within these sizes. Because while it is possible to join seams between panels, it is a lot more efficient and cost effective to avoid it.
When I make enclosures or build units for our displays at shows, I always go to Bunnings and sit it the materials aisle surrounding myself with what is available to me. Sometimes you can figure out better ways of building something or find a cheaper option you may not have thought of before.