That is an interesting project! I like the idea very much. I have a few thoughts that might help, considering that you want to keep this setup entirely portable but it needs to be stable:
1. When you get the exact location of the blocks established with the fencing, chip out a slight depression in the shale/dirt. Doesn't have to be much, just 1/2-inch or so, for the block to sit in. It will not be dislodged without a lot of force, then. It would have the added benefit of raising the wood boards up by the same depth, making the lateral gap between board and fencing tighter.
2. To make the center tine sit tighter in the blocks, I have a couple of ideas.
a. There are 12-inch x 3/8-inch spikes available at any hardware store, like this:
You could use those to drive down the center hole next to the tine (if it fits), instead of wood wedges. Not ALL the way down, leave a stub so you can easily snag it and remove with a hammer later when you want to move the blocks. It would drive into the shale/dirt below (much stronger than your stakes), making your blocks even more stable. OR...
b. Find some rubber tubing or thin-walled plastic pipe that fits tightly inside the center hole of the block, but still big enough for the center tine to fit loosely into. Epoxy that tube into place.
3. With a little work, you could modify the blocks (no tools needed) to make a second hole for the outer tine. Cover the tine in oil or vaseline, and wrap it with saran wrap, then place it upright into the block - brace it so it's perfectly plumb. Mix a little cement and fill the outer notch where the wrapped outer tine sits. Press a board or something against the open side of the notch to hold it until the cement dries. Let it cure overnight. Then pull your tine out of the block, remove the saran wrap, and voila: perfectly sized hole that's snug but not stuck.
You could probably do that with the center hole and tine as well, but you should use a high-strength, pourable cement made for filling cracks and thin applications. Just fill the hole completely, then push your oiled and tightly wrapped tine down through the cement. Brace it and let it cure. Pull the tine out, and there you go.