paper/poster board is too thin, it'll be really hard to make structurally sound.
i got Mat Board from Michaels Arts and Craft, Studio Decor 24in x 36 in.
plan on using 3 sheets for 2 buckets barring messups.
you won't find it with the regular cardboard/craft paper/corrugated but rather with their picture/painting frames.
edit: just double checked my Mat Boards thickness and it clocks in at 1.58 mm or 1/16 in.