I would buy the smallest concrete paver I could at any landscaping store, drill a hole through it at one corner and put a bolt and chain through that hole so you can pull that paver out of the pit if you ever need to. Set the paver down on whatever you have (crushed stone OR gravel), and then set the sump pump on the paver. A sump pit will have a grate around it's base to sift out anything too big to go through it, but it's better if no stones go through it at all.

Elevating the pump a bit by setting it on a paver will ensure you don't suck up any stones. The water in your sump pit will be 2 or 3 inches higher than normal, but that doesn't matter.