I'm pulling ideas for the base currently. the columns are roughly 5.5" wide 9" deep 35" tall. I'm figuring on doing along the lines of 12x14 for the base, but not sure on ideal weight. I will take what has been said into consideration for this. The top plate will be just shy of overall dims of the ultras, 8x10.5 im sure once the plate is on top, i will get a better feel for the stability.
Keep the weight low and it shouldn't have any problem with tipping, especially if you use something like Blue Tac to attach the speakers to the stands (it's a rubbery material that can be removed without damaging anything). Lead shot isn't expensive and it's something that has been used for a long time to prevent tall, skinny things from tipping over. If you create a chamber at the bottom and add enough lead to double the weight of the speakers, you should be fine. If you think of the whole assembly as a post, it's easy to push it over when the weight is evenly distributed but as soon as you attach it to a wider base, it will stand on its own.
In Physics class, we needed to determine the center of mass in various objects- if the weight is evenly distributed, it will be in the center of the object- make a diagram of it and draw a line from each lower corner to the opposite corner at the top. The intersection is where the Cg (Center of Gravity) is located. Widen the base and the spot will move toward the bottom. Add weight at the bottom and it drops even lower. The angle where the Cg moves outside of the base (assume that gravity is the y axis) is the angle that will allow it to tip- any less and it will want to go back to where it was.
Let's use a 5'post with 1/10 of its volume filled with lead. If it's White Pine, let's use 25 lb/ft³ as its density, so a 5' post with 1ft² as its cross-sectional area makes it easy- it's volume is 5 ft³ and its weight is 125lb. Remove 10% of the bottom and replace it with lead (density of ~708.5 lb/ft³) and we have removed 12.5 lb of wood (1/2 ft³) and replaced it with 309.25 lb of lead. It would be hard to topple it under normal circumstances, even if half as much lead were used.