The best design I have found so far is this link:

http://www.maryjanesfarm.org/Recipes-Patterns-Instructions/horse-troug h-gardening.pdf

She is using oval troughs, I have 5' Round trough. She uses 4" drain pipe to support thick rigid plastic grid to hold up the soil. I would think you would then put some felt weed barrier on top of that to prevent soil from falling through, but she did not say that. Seems pretty simple and effective. She showed pretty impressive growth in one month.

SOIL: I have been told by local organic gardeners to use a 50/50 mix of compost and local soil. I am wondering if with a SWC, there is an issue of compacting with this type of mix. She recommended 1/3 #2 vermiculite, 1/3 peat moss, and 1/3 compost/soil (screened if
necessary). She supplements with mild manure tea or liquid seaweed fertilizer by pouring it down the filler pipe during the growing season.

Any concerns with this design or improvements?
Can I grow year around in my zone or would I have to drain the water at certain overnight temperatures. (we average 17 days under 32deg)

I recently purchased 2-8' tanks and 2-6' tanks. I placed the tanks on cinder blocks to raise them higher. I used 12 blocks for the 6' tanks and stacked them by 2. I used 8 single stacked blocks for the 8' tanks and left them lower for a tiered effect. Then I move on to order the sheeting.

When I called the maufacturer of the polypropelene, Ametco, they sell the sheets in 4x8'sections, cost is $144 per sheet, and they ship UPS after they cut the sections in half for ease of shipping. I have been searching for alternatives for the sheeting due to the cost and the time it takes for shipping.

I agree that the problem are the holes in the sheeting. How would the holes keep the soil from filtering through over time? Eventually the soil would fill the bottom section.

I am stumped to figure out a different medium for the sheeting. On one soil stained hand, if I use barrier cloth then any type of 'false' bottom that would support the weight of the soil should suffice. On the other soil stained hand, that raises another concern- what is a safe material that will not break down over time?

I am researching the idea further but wanted to let you know that I am also pondering similar questions to yours.
I will update you about my findings.

Happy Gardening!