Hardener either 205 or 206 which will give you more working time. Brush the holes with the liquid epoxy before mixing the colloidal silica in until it is about as thick as peanut butter. Place tape like duct tape over the holes on one side. Fill the holes with the thickened epoxy and tape over the remaining side until it is set. This will prevent sagging. I sometimes tape a piece of plastic over holes as the epoxy doesn't stick to it and when set it will leave a finish that requires little sanding.
I looked it up. what about 404 high density filler? I'm going to re-tab it with biaxial. Would 404 work for both applications?