Over they years I have made several slide hammers to compact soil. I usually use a piece of 2 inch round stock and drill a center hole in it with either a lathe or mill oversize to whatever rod I am using for the slide. I weld a large washer on the bottom of the round stock to keep it from interfering with a weld on the plate. I drill a net hole in the bottom plate and press in the rod. Then I burn it together with a welder and a nut on the top to keep the hammer captured.
I have made different styles, different weight amounts, and length of slide, and size and shape of the bottom plate. Anytime they want a different shape I just crank out a new one.
I also made a small plate compactor using a pneumatic chisel. We had to decrease the surface area so it would work properly.