I used U bolts on mine, work a treat with spreader plate (supplied) and Sikaflex. Also from FleaBay. Single bolts as described above possibly better from an aesthetic perspective and less drilling!
Test run was yesterday, kids loved it and a bit of a learning experience for me, the SIB doesn't grip anything like a rigid boat, I knew it wouldn't, but wasn't quite prepared for quite how slippy it was especially since it had performed very well when tight cornering etc.
Also, tubes are very draggy, a knee board might be better with a smaller engine? Not that the kids care, they'd happily be dragged around at 2 mph so long as mum and dad ain't around to pester them!