1. Using an angle die grinder with a 3M 2" 24-grit Roloc disc, grind the rough edges, high spots and spreader marks down until it's close to your desired shape. Use more fiberglass-reinforced filler in areas that need more than 1/4" of fill. Don't waste time trying to make the shape perfect with the reinforced filler.
Now that the fiberglass and reinforced filler for the top of our multi-composite enclosure have cured overnight, it's time to start the shaping and finishing process. Using an angle die grinder with a 3M 2" 24-grit Roloc disc, start grinding away the excess fiberglass and filler starting at the outer edges. Always use a dust mask, gloves and eye protection when grinding fiberglass with power tools. (Safegrip latex gloves by Microflex are great for this process because they are thick and have long cuffs to keep the fiberglass dust from getting under the edge of the glove.) Grind down the high spots and spreader marks while frequently running your hand over the surface to monitor progress. All masking tape should be removed, especially if there is any trapped under the fiberglass or filler, but avoid gouging the MDF speaker baffle. As you are grinding, keep in mind that the goal is to create your intended shape, not to remove all of the filler. I often hear from people how much they hate sanding and grinding. While it's far from being a pleasant process, it's one of the most critical steps to create your masterpiece. I relate the process to sculpting or carving artwork, but our medium is fiberglass and body filler rather than marble. You're the artist who visualizes the shape and it's up to you to create it. The shaping and finishing stages are where men are separated from the boys, because it determines whether the end result looks like professional work or a grade school project.
2. A batch of Rage Gold body filler with the proper amount of cream hardener. Lightweight body fillers generally require less hardener than fiberglass-reinforced fillers.
If your shape has any low spots that need to be filled more than 1/4" thick, do so with more Duraglas/Everglass fiberglass-reinforced filler. Grind and fill with reinforced filler (and repeat) until the top is close to your desired shape and any low spots are less than 1/8" deep. The surface may still be rough, as long as the overall shape is very close to what you want. It is not necessary to hand sand the reinforced filler smooth.
To take the project to the next level, we will add a layer of Rage Gold body filler. While there is a wide variety of body fillers to choose from, I, along with many autobody professionals, use Rage Gold by Evercoat for the finishing stages because of its ease of application and sanding. All body fillers are definitely not created equal. Premium body fillers, like Rage Gold, help create better results by producing fewer pin-holes, curing consistently and sanding easily. Although the premium fillers may cost as much as $20 more per gallon, the time and effort saved, along with the superior end result, justify the higher price in most cases.
3. A thoroughly mixed batch of filler will have a completely uniform color.
Before you start the final filler stage, tape and protect the areas that need to stay filler-free. It takes a lot less time to tape an area off than it takes to sand off hardened filler. Scoop out an appropriate amount of Rage Gold onto a mixing board (a clean, piece of MDF or cardboard will do) and squeeze out the proper amount of cream hardener. In general, finishing body fillers require about half the cream hardener that fiberglass-reinforced fillers need for the same 10 to 15 minutes of working time. If you don't have a lot of experience mixing body fillers, you may want to mix a small test batch first to get a feel for the correct hardener-to-filler ratio for the climate you are working in. Mix thoroughly until the entire batch is a uniform color. It's a good idea to use one spreader to mix with and another to apply the filler, so that unmixed filler can't make its way onto the project. Plastic spreaders are available in several different sizes and the right size should be selected for the given application. You can also cut the spreaders to custom shapes and sizes to fit your needs.
4. Apply a layer of Rage Gold body filler over the top. Wait for the filler to reach the "green stage" and then immediately start shaping the surface with 36-grit sandpaper. | 
5. Just before the filler is ready to sand, it will "roll" off with 36-grit sandpaper. It will be in the "green stage" in a few moments. | 
6. The filler is in the "green stage" and ready to shape when it starts sanding off like grated Parmesan cheese as shown here. Quickly remove spreader marks and high spots during this stage. |