One could cover it with one of the kinds of substance that it’s not prohibited to dig - the “non-genuine” soil.
The Pali word for soil, paṭhavī, also means ground or earth. Thus the Vibhaṅga distinguishes which forms of earth are and are not classed as genuine soil:
Pure loam, pure clay, whatever is mostly loam or clay with a lesser portion of rock, stones, potsherds, gravel, or sand mixed in, is classed as “genuine” (or “natural”) soil (jātā paṭhavī).
Whatever is pure rock, stones, potsherds, gravel, or sand, or any of these with a lesser portion of loam or clay mixed in, is earth classed as “ungenuine” (or “denatured”) soil (ajātā paṭhavī). Also, burnt clay or loam—according to the Commentary, this means soil that has been burnt in the course of firing a bowl, a pot, etc.—is not classed as genuine soil. As for heaps of loam or clay that have been dug up: If they have been rained on for less than four months, they are not classed as genuine soil; but if rained on for four months or more, they are. At present, irrigated soil would count as “rained on” as well. Also, the layer of fine dust that forms on the surface of dry soil as the result of wind erosion is not classed as genuine soil.
(Ajahn Thanissaro: Buddhist Monastic Code vol. I ch. 8)
But if nothing of the sort was available, it would probably be best to just break the rule and then confess it. I mean we’re not talking here about the type of transgression that would be kammically unwholesome.