It is very difficult to enlarge a perfectly round hole with a dremel or roto zip, especially if you have to enlarge several holes. No big deal if the holes are covered by a collar I suppose, and those tools are least likely to cause splinters. I've successfully used a hole saw freehand by letting one side dig in first then rocking it perpendicular to the
wood. A much more reliable method is to make a guide block as previously described. If you can't go all the way through, forget the hole saw. The guide drill extends beyond the barrel. Use a router, bushing, and guide block, a forstner bit and guide block or I suppose you could cut a stepped bit. Just be sure to firmly clamp your guide block if using a drill and go slowly when you first enter the
teak.
Brett