Hello; to keep the base from sinking into the ground there are several options. You will want something with a large footprint, that is strong and will not rot or attract bugs. They sell pre-cast cement bases for things like decks at the big hardware stores. These flare out at the base and are made to accept a 4x4 post. a large square of metal will work. A few solid cement blocks with a square of pressure treated plywood between the blocks and the jack base should work. The only wood to use directly on the ground should be pressure treated, but that would be a last choice for me.
A hole in the dirt could be filled with a cement mix to form a footer type base for the jacks. If you have enough room to work(dig). This may be the best choice. This should be confirmed, but I think it is possible to pour the dry cement mix into such a hole and either allow the soil moisture (if there is enough) to set it or add a small amount of water?? I have done this in places where the set strength of the cement mix was not critical. Adding too much water can weaken some cement mixes. I have seen such small cement footers with stacks of cemented blocks under floors that support houses, so that is another option if the jacks you seek are not found. might be too hard to do in a confined space.
I have seen floor jacks at lowes, but am not sure that they had lengths as short as you want. I suppose they can be cut to length??
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