I'm looking for an inexpensive way to put shading cloth over my garden

How about using a bit more stable of a structure like this?

The above idea is great, and to deal with the rocky soil, just buy some buckets and quikcrete and "plant" the PVC uprights in a bucket of quilcrete.

Good idea about the quikcrete. But make sure you attach your shade cloth well to some crosspieces of lumber. Rolling the fabric around a 1" by 1" and anchoring that to your fence or whatever would help. Just anchoring at the corners isn't enough.

Maybe use old venetian blinds or mini blinds for shade. Or better get some of the free pallets that seem to be available everywhere and nail up a shade from them nailed to 2" x 4"s . It does not look like something flimsy is going to do the job.