Get some nylon rope into the arrows you are currently shooting.
The yellow kind usually has three individual bundles twisted together.
Separate one bundle from a piece of rope about one inch longer than your finished arrow, and use two bundles inside the shaft. you will gain about 100 grains without changing the spine.
Usually, I feed a piece of fishing line thru the insert all the way through the shaft and connect the rope to that. Simply pull the rope into the shaft and when it gets to the insert, just pull hard enough to snap the fishing line. Using a small screwdriver, stuff the extra length of rope you have left far enough in to push your nock back in place and shoot it.
Cheap and quieter than weight tubes.