It's just my inexperienced opinion that a plain layer of gravel (3 to 6 inches deep) would suffice for panfish beds. Mine seem to like it fine. I put in creek gravel (from the creek) that consisted of fines all the way up to baseball size rubble. I had bedding activity the first year the fish were in the pond.

Tires might be better suited for hiding structure, especially when stacked, and maybe even channel cat hidey holes.

I have also read about folks using saucer sleds and satellite dishes to create individual beds. Maybe there is some advantage to making solitary areas, but I don't know how much better it would be compared to a large patch of gravel (or individual smaller patches). If I were to try and make tire beds, I would put enough sack-crete in them to fill up the side that would be down, drill several big holes in the up-side and completely fill the center with a gravel mix...maybe even stack rocks around the perimeter to add to the seclusion.

Just my two cents...

Fish on!,
Noel