I had the pdf of the spirit board from WotC's site (which you can find here) printed on cardstock. My home printer wasn't up to the task, so I snuck it into an order with the printer I use to do my zine pages.
I used adhesive spray to affix the cardstock print to some black foam board, then trimmed it with a hobby knife.
The trimming process will inevitably leave some shedding, ragged edges. I smoothed the edges of the foam board by quickly moving it around near a candle flame. The flame melted and sealed the edge rather nicely for such a primitive technique!