The cheapest and coolest looking mast float i know of is your mast! haha...Get some silicone and a old cooler and go to town!
Remove mast head(drill out rivets)(top peice)
Take one peice of the cooler and place it flat on top of the mast and push on it a little to make an imprint of the mast. Cut out the cooler so that it fits snuggly inside the mast. remove peice from mast and cover in silicone on the sides. Slide peice back down inside the mast about 8-12 inches. Make sure transition from cooler to mast walls is siliconed very well (you may want to add some more for safety and insurance. Apply silicone to the sides of the mast head and press back into mast. Re-rivet the mast head back on and check for spots that are not filled with silicone adequately.
This is what i did on my boat that almost sank because of a leaky mast. I went out and flipped it the weekend after i did this work and left it on its side for 20 minutes and didnt take on 1 drop of water. Its easy, cheap, and looks a hell of alot cooler than a mast bob!

If you dont understand what im saying i can send pics