big board advice needed

[bluegrass]

I'm building a 4'x2' board. I'm working with a .080 sheet of acrylic in a frame, and a motor from a new model 620 which didn't do all that hot on a 620. The sheet of acrylic seem to be conducting vibration pretty well but it seems to only power about a 1/4 of the board. I want to make sure this will work before I check into geting a motor from Joe and order a field cover. Would a larger screw and spring improve vibration to see if this works. Then there is the trought of using 1/8" thick fiber board with an eraser board finish on one side. Both options are much cheaper than sheet metal and easy to work with. Any thoughts and or advice is welcome. Thanks Brian