I agree with Chris, that is not a 16:1 downhaul, it is an 8:1 made by cascading a 4:1 with a 2:1.
Even in their description, Item "D" is called a 4:1 Downhaul block.
The second turning block is not needed. Like Chris said, most people use a clam cleat and preload the hell out of it.
Make the lines long enough to use the old Hobie 14/16 method of hold the end of the line and standing on it.
You may even have to redo it after the first race to keep it tight.
Most of us on the 18sqs used 8:1 downhaul. I and some others used a straight 8:1, while some used a cascaded 4:1, 2:1.
I even cascaded my 8:1 into a 16:1 for a while to see what it was like. It made adjusting very easy but I didn't use it very much.
How are you attaching the downhaul to the sail?
Some of the 18sqs used the big hooks. Some bolted double blocks to the clew plate.
I added an extra clew plate to the bottom of the sail with a single cheek air block on each side and I bolted a single cheek air block to each side of the standard clew plate and fed the line every time I rigged. The lower plate and blocks would pull the wrinkles out of the lower section of the sail. It wasn't very fast but it was clean and light.