I made ours from some 100×50 timber that had previously been false rafters in our bedroom. Dug away top soil, put in some hardcore and levelled four pads, put a 150mm concrete block on each pad as a footer and to keep the wood frame up out of the damp. The frame itself is roughly 2000x1500x1000 and can fit two full loads from the local tree surgeons, roughly a tonne a load. Slatted floor and sides, roof was some spare board covered with roofing felt. I was going to bolt it to the house wall but it’s heavy enough that it doesn’t move.
The store is in two halves, order a new load when one half is empty so that the new delivery has time to dry.
I think the whole lot cost about £30 and a day or two’s work.