Source: FM 3-21.75, Figure 6-3. Prone Position (Hasty).
FM 3-21.75 The Warrior Ethos and Soldier Combat Skills field manual provides the following narrative for the construction of a " Hasty Fighting Position." Hasty fighting positions, used when there is little time for preparation, should be behind whatever cover is available. However, the term hasty does not mean that there is no digging. If a natural hole or ditch is available, use it. This position should give frontal cover from enemy direct fire but allow firing to the front and the oblique. When there is little or no natural cover, hasty positions provide as much protection as possible. A shell crater, which is 2 to 3 feet (0.61 to 1 meter) wide, offers immediate cover (except for overhead) and concealment. Digging a steep face on the side toward the enemy creates a hasty fighting position. A small crater position in a suitable location can later develop into a deliberate position. A skirmisher’s trench is a shallow position that provides a hasty prone fighting position. "When you need immediate shelter from enemy fire, and there are no defilade firing positions available, lie prone or on your side, scrape the soil with an entrenching tool, and pile the soil in a low parapet between yourself and the enemy." In all but the hardest ground, you can use this technique to quickly form a shallow, body-length pit. Orient the trench so it is oblique to enemy fire. This keeps your silhouette low, and offers some protection from small-caliber fire. The prone position is a further refinement of the skirmisher’s trench. It serves as a good firing position and provides you with better protection against the direct fire weapons than the crater position or the skirmisher’s trench. Hasty positions are further developed into deliberate positions that provide as much protection as possible. The hole should be about 18 inches (46 centimeters) deep and use the dirt from the hole to build cover around the edge of the position (see attached example).