I use a 200 ml milk bottle with 4 drilled or burnt through holes in the top of the cap and another 4 drilled through holes in the side of the cap. For a more bubbly solution you can burn through 4 holes below the cap as shown in the third photo below.
Fill up the bottle with a suitable liquid detergent and a stone.
Lastly, put it into the water cistern.
Have fun flushing with the Shit Bowl Assistant :)
(I started using this on 8th August and today {24th August} the toilet bowl remains clean)