Weight, its your friend in this sport. Where the weight is located is also important. Pushing as much weight as possible below the axle height will give you a much more maneuverable bike. So what you can do is drill a hole in the frame and fill parts of it full of lead or concrete. Some builders will bolt on lifting weights this is nice because you can easily move the weight on the bike and or increase decrease the amount.
What we have done here is taken two steel hemispheres drilled a half inch hole in the center of both hemispheres. In one hemisphere we welded a piece of 1/2" all thread through the sphere. Leave a few inches on the inside and enough on the outside of the hemisphere to mount it to the frame. Then weld the two hemispheres together. Next, ground off excess weld to create a smooth looking sphere. A tube was welded to the sphere for a "wick" to be added later making a classy looking little bomb.
Now for the awesome nasty dangerous part. Filling the hollow sphere full of heavy stuff. Using a gas forge we melted lead pellets and then poured the melted lead into the sphere. A funnel is of great assistance during this process. Ours is an Ace model for about three dollars. This created a 25lb ballast. There was still room for more weight so we drilled a hole in the bottom tube of the frame and filled that full also. Do not try to cool these parts down with water. Just let them do there thing. It may take an hour or more. If you do not have a gas forge at your disposal an oxy/acetylene torch or MAPP Gas torch will also get the job done. If you go down this road please where a proper respirator and do it outside if possible.
The excess all thread that was left protruding from the sphere was enough to safely mount it in a vise for the pouring of lead. After the lead has cooled we drilled a half inch hole in the frame and mounted the ball. You can bolt the weight on using a nylock. We chose to cut the all thread down until only .125" was protruding out of the frame, and then welded it on.