How hard do you have to file or sand? How do you get the Marine Tex inside the trunk? A stick that you stir paint with? That would work for the flat sides but not the rounded ends. And if you put in too much would the dagger board not fit?1) What is your lake bottom and shoreline like? That dolly may dig right into it. It may have been sold through the years through the Alcort or AMF catalogs or Windward Leg Class Association newsletter. It would work well for concentrating the full weight of the boat on a gudgeon that is held on by 4 small screws.
2) Here is one of our blog posts on how to repair resin chips in the daggerboard trunk.
Small Boat Restoration: Daggerboard Trunk Repair and Leak Test
It takes a longer file or sander to reach inside the trunk and file away loose edges of the chip and burr a surface for the epoxy putty to adhere to. We found a long diamond ceramic tile file at Lowes that works great, it is flat on one side and round on the other. We have also used 60 grit sandpaper taped to a paint stick, or the 4 in 1 file to get to the first few inches inside the trunk. Alan has a extension bit for a drill that holds a Dremel sanding drum, the best method.
For your repair Marine Tex epoxy putty (White) would be the easiest route to go, available through Jamestown Distributors.
Marine-Tex is a high strength epoxy putty that hardens like steel and is sandable like wood. Use it to repair holes, cracks, dents, worn out surfaces and damaged substrates on your boat and beyond. This unique putty bonds and fuses ceramics, fiberglass, plastics, wood, metals, ferro-cement and porcelain.