1/4 inch MDF (Home Depot)

you can also use a underlay plywood fom howe dwpot which i use to cut 1/4 inch

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Underlayment is one of the worst materials to use. It’s one of the lowest quality of plywood you can get and filled with all kinds of unknown materials, inconsistent not only from one sheet to the next but even on the same sheet.

It’s designed to provide a flat surface for tile and such.

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You can usually get 4x8 sheets and with four cuts at 19" gives you 5 - 48 x 19 inch sheets. I would look for Revolution Plywood as while it has some bad spots they are less problematic than usual and way easier to cut. And near the bottom in terms of cost.