do a search loh.. I made a DIY tubby even before the tubby started being produced for sale... even uploaded the schematics of it and costs etc etc...
not to mention the risks involved... I got electric shocked twice because of stupidity...
IMO, not worth the trouble... but if u on ultra tight budget like myself can give it a wack if you are willing to accept the risks involved...
BTW, my DIY auto-top up been working for >1year liao no problems so far...