The fat O’Brien vortex with the fin... we use these.
https://www.ebay.com/i/282861791501?chn=ps&norover=1&mkevt=1&mkrid=711-117182-37290-0&mkcid=2&itemid=282861791501&targetid=484978871504&device=m&mktype=pla&googleloc=9013386&poi=&campaignid=1497793369&mkgroupid=60383707400&rlsatarget=pla-484978871504&abcId=1139456&merchantid=6296724&gclid=EAIaIQobChMI7_aIrPbs4wIVB7vsCh09GwUfEAQYByABEgI5FfD_BwE
And they work like a charm. Skiers in the pic from left to rt are ages 6, 5, 4, 6