1 pound nut meal/flour (almond and cashew work well, others undoubtedly do too)
Put 3 cups of water in blender. Add nut meal. Stir, then blend briefly just to amalgamate. Add enough water to reach the 6 cup mark on the side of the blender. Blend for 5 minutes. Use immediately or freeze. (I freeze one cup at a time in pint-size Ziploc bags or pint jars.) Straining is optional, and a regular blender works just fine--no need for a Blendtec or Vitamix.
If you want to make smaller quantities, 2 cups meal to 1.5 cups water will get you about 2.5 cups of a nice heavy cream. Add more water if you want to thin it out.
I'm never organized enough to soak nuts for hours before blending them into cream; pre-ground nut meal solves this problem nicely. It can cost a bit more than raw nut pieces ($11/lb vs $9/lb at Nuts.com), but the convenience is frequently worth it.
Nuts.com has really good prices on certified GF nut flours. Shipping for five or ten pounds isn't much more than shipping for one pound, so order in bulk!