We just rebuilt our JV pens with concrete borders instead of ties and it is much cleaner and easier to maintain. It looks much nicer and will last forever where ties break down and move. We did it with donated labor and concrete at cost which made it affordable. If you are going to use clay make sure you lay bricks down and only put a very thin layer of clay/dirt mix, prevents them from getting giant holes at the landing spots. Once we get a few more $$ we are going to go to all concrete and astro turf. This will last much longer and require much less maintenance.
BTW plates are already on concrete pads with turf on top with painted plates. Much cleaner and no maintenance.