There are a lot of materials that work as a base for the layout once the table has been constructed including homasote, pink fibreglass sheets or grass mats.

I decided to check out indoor/outdoor carpeting as a base.  The cost is reasonable, it deadens the sound and if you choose carefully, you can find a mottled green colour that in spots works fine as a grassy area.  You can glue to it, easily nail through it and it does not have the dust of homasote or fibreglass.

Indoor/outdoor carpetting is also easy to cut to fit and can be easily attached to your table with a staple gun.

Give it a try.  It certainly worked for me.