I use old socks filled with sheep muck, suspended in the middle of a spare water tub. (Use an old incinerator lid for the top and have the suspension wire fastened to a stick across the incinerator lid chimney. Lift the lid and swirl it all around once in a while to keep it doing its' stuff. Generally have a few tubs on the go (old large plastic chemical drums with the top sawn off). Seemed to work as well on my toms last year as commercially produced stuff.
It do pong 'a bit' though, true, definitely not one for anywhere near habitation!
very old joke:
"What you stirring that sheep muck inter t'watter fer?"
"Puts it on mi tomatters."
"Oh aye? Prefers mayo on 'em, misen..."
I'll get me coat.