You could do a shaker panel out of birch 3/4" plywood (for the centre) and maple 1x3 or 1x4 frame (depending on how big the overall panel is so that it works proportionally) ... Stain or paint it to match your vanity. If this is going to be too big and possibly warp (I've no idea how big your tub is) then you could do one fixed centre panel and removable matching panels on either side depending where you need access. Hope this makes some sense. Depends if you're up to making cabinet door panels as that's basically what's am describing. I would use this sort of catch to attach to the framing ... http://www.richelieu.com/en/catches-...c?idGr=1004730
Other than for looks and durability (the edge of plywood is not good for taking dings) the reason for the shaker is so that you don't have the plywood edge exposed to potential moisture issues from people getting in/out of the tub.