You don't say where you are, which makes it a bit difficult to recommend specific materials. I'd try medium density fiberboard (MDF) here in the US. This material is sometimes available in different grades/densities, but Home Depot usually carries it. Other types of manufactured (non-solid wood) panels are available, but are often too flexible and warp too easily. MDF of the correct grade is composed of compressed resins and very small wood particles, formed under about 50,000 lbs/sq in of pressure. It has a very flat surface and is very stiff. You'd be best off applying a laminate to prevent moisture absorption and consequent warping. Edge banding is probably also a good idea. Save the hardware from your current baseboard and fit it to the new base.
Lee