Here's a picture of the progress on the plug for the Borealis that will fit the Sprint from ICE.
In the wheel well you can see how much the plug has been widened, 1" each side, to make it fit the Sprint. You can see the different colours of the fill used to build up the shape. I have been using left over gelcoat and resin with a filler. I mix up these leftover amounts with a very light weight filler to make a paste that I trowel on to the approximate shape. Once it hardens up I can sand this quite easily as its very soft. Its not tough enough and too porous to take a mould off so I have to encapsulate it in fiberglass and then use a harder filler and sanding gel to get to a moldable plug. You can see the fiberglass cloth hanging off the bottom edge of the plug. The reason for the cloth is to hold it all together as I have had the misforutne to have the soft filler pop off a plug when I dropped it. It has such a high filler loading that its bonding strength isn't too good.
The plug still has to have the recess for the rear wheel cover added but the basic shape is all there now.