For filling pinholes and similar imperfections, I prefer bondo spot & glazing putty. It woks better than Squadren in my opinion as it has less of an issue of shrinkage -- so you can usually get away with fewer coats. For large gaps, I use 2 part epoxy putty . Both items i have mentioned are available at any Walmart and are very inexpensive . The epoxy putties come in many varieties, but I like the stuff that comes in a roll with blue on the outside and an inner core of white. When they are mixed together, just roll the material in your hands and press into the gap removing as much of the excess as possible. It can be moistened with rubbing alcohol or water to make it more pliable. Work as quickly as possible as it heats up and cures very quickly. When it has cured, it can be sanded, filed, drilled and cut to any desired shape.