hau Dan Long, I could not find your blog, the URL in your signature takes me no where. So I have no way to see what you are doing.
For sunken mounds it is customary to reserve the dirt dug for the topping layer.
I like to mix the dirt I dig for a mound with lots of green material and brown material before using it as the top coat.
I also use a 3-4 inch thick layer of straw over any fresh mound to keep the dirt moist while the cover crops sprout and grow in.
When you are building a fresh mound with the sunken method, you should end up with around 12-18 inches of above grade height of the mound.
If you are using enough wood in this method, the dirt dug out should, with the mentioned amendments, be enough to put on a topper layer.
It is of utmost importance to be able to stuff every nook, cranny and crevase with greens, browns,
compost and compostables before you put any dirt on.
The reason is that these materials are what will get the mound working as they are supposed to work. If you need extra dirt, then you didn't use enough of the other materials during the build.
If, you do everything right and do find you need more dirt, then the place to get it is from the swale you will put right next to the sunken mound.
Planting some of the right types of plants and flowers will help curtail mosquitoes.