This is a cool idea from Nimish!

"I found these Old Castle Blocks @ Lowe's.  You can place them at each corner and slide a piece of wood between them to make the garden bed.  The nice thing about them is that they have a ½ inch hole in the center for rebar to hold them in the ground, but it also fits ½ inch conduit (over the rebar) quite well.  I ended up stacking 2 of the blocks high.  I have ¾ inch conduit sitting on top of the ½ inch conduit because I think it may be more stable.  There is a little bit of wiggle between them."