Thanks Wayne again for the input. I was thinking the base would have to be nearly as large as the speaker bottom for stability.
I've attached a pic of the problem area. The current speaker is 11" wide, the 4pi is 18.5" wide (and will have greater toe-in). I cannot move my entire setup over to the left to accommodate (stairway opening on the other side), so some portion of the 4pi has to sit on the raised floor.
I think I have come up with a solution. Basically a platform the same size as the speaker bottom, with four legs on each corner extending downward. One rear leg will be 4" shorter and sit on the raised floor, problem solved.
Any other possible solution anyone sees most welcomed.