There're a lot of pages in the internet with tips to build a monopad:
1, 2, 3, 4, 5, and 6.
But there is a way far easier. You just need a cheap tripod and a broomstick. You just have to take the head off the tripod and spin it on the broomstick.
The lever to tilt the head incommoded me, therefore I've cut it.
Certainly, you can still use the head with your tripod.