I have found 1/4" X 4 1/2" X 96" the toe kick boards used in cabinet making for as little as $2.00 at wood salvage stores. The ones at the salvage stores are often finished on one side. I have found the veneers to be very nice walnut, cherry, oak and mahogany, the core is MDF. The veneers may be thinner than proofgrade and they are not masked. If you don’t have access to a surplus store they are available through other lumber vendors. https://www.menards.com/main/home-decor/home-accessibility/kitchen-laundry-accessibility/shop-all-kitchen-cabinets/quality-one-trade-96-x-4-1-2-unfinished-cherry-toe-kick/otk96toekickcherry/p-1444451722539.htm
I have been making use out of 5mm underlayment from home depot. Just be picky and use hold down pins.