This recipe is pretty easy to scale up. I usually make a double amount, and store the extra in a glass jar in the fridge. I find that the garlic and ginger amounts listed in the recipe to be a bit weak, so I usually use a whole head of garlic and a whole hand of ginger.
Mix soy sauce, sugar, mirin, corn syrup, and salt together in a medium-sized sauce pan. Heat over med-low, until the sugar has thoroughly dissolved. Take a few spoonfuls of the liquid, and mix with the cornstarch in a small cup to make a slurry. Slowly add the starch slurry to the saucepan and mix well. Add the ginger and garlic, and simmer the mixture uncovered over low heat until slightly thickened. Allow sauce to cool, and then place in glass jar.
Since this is a pretty sticky sauce, I find it helpful to place a sheet of plastic wrap over the top of the jar, then place the lid over the plastic wrap. This helps keep the threads of the jar lid clean.