To make this rig, three suction cups (Amazon) are attached to a tripod’s feet via an aluminum bracket, marine sealant, and these ball head pivots from Ebay. The illustration below should give you some idea of how it all fits together. Further assembly info can be found in this set of pictures.
Of course this mount looks quite sturdy, but if you want to see it in action, check out the video after the “read on” link.
This DIY mount isn’t exactly cheap, as the suction cups, and pivots alone will cost around $45. Add in the tripod and aluminum adapters, and you might be talking about $100 or more. I was curious to see how much a purpose-built tripod base would cost, and after some searching found this one from Amazon for $275. So you could possibly save some money going the DIY route. The purpose-built one does supposedly holds up to 9 pounds though, so if you’ve got something bigger than a GoPro, it might be worth it!