Morning Chris,
I am in the middle of building a three row planter right now using the JD 71 Flex planters. What I am doing is using a 6x4 tool bar that is 7' long.
I purchaced 3"x4" angle iron and I am cutting the bottom pieces 12" long and drilling them on the 3" side to bolt up to the tool bar using U-Bolts that I had made up. I am drilling the 4" side for the standard lift pins.
I am then taking two of the 3x4's and cutting them at 14 1/2" long, these will be also drilled to accept the u-bolts and then they will be drilled to accept a top link pin. These will have a piece of square tubing in between them welded and drilled to accept a foot post that I can put down to help balance the planter when put down on the ground.
I am cutting and drilling today so when I am done I'll try and post some pictures of the mock up so you get a better idea of what we are doing.
If you search e-bay for corn planters you will see a lot of one and two row planters that might help you, looking at these is what gave me the idea to do mine this way. Hope this helps.