The hands were made from a children's size pair of mechanic's gloves, in black.

Yeah, they were kind of hard to find, but the tools section at Lowes had them.

The fingers were extended to match Groot's huge hands with a bit of polycarbonate cut from a fluorescent light guard tube.  This gave us a rigid structure, but not too rigid as to present a risk of injury.  

The results are actually usable for grabbing things. 

We also did not attach them to the shirt at inside of the wrist, so his hands can be taken out of the gloves for things like eating.