If you do a search on possum board you will find lots of good info on the forum discussions on what they are for and now to make one, as in part of the post below - I hope you don't mind ken my reposting part of your post.
Ken posted:
"Dulcimer on a table - whether 5/8" or 3" thick -- is better than dulcimer on the lap. The lap is soft and absorbs sound. Tables are hard and reflect sound making the dulcimer louder. However. Dulcimer suspended even 1/8" above a "table" gives you even more volume because it frees up the entire back of the instrument so it can vibrate."