Great system you have BTW. From your pics you'll have enough room in front of the sump for the other IBC/DWC. If you had placed the sump away from the GB's initially, it could have been used as a DWC. At any rate i see you still have some surface area exposed in the sump which can be used as a DWC. If you still need to add a new DWC with the unused IBC, place it alongside the sump at the same height, pipe them together at some point below the waterline so that water flows from one to the next, and place the pump inlet in the new DWC and outlet going from there to the FT. So it will be FT going to GB's, overflowing to the DWC/(DWC's?), and returning to FT. The water levels won't fluctuate that much because the combined sumps(DWC'S) will have more volume than the FT and GB's combined, also the GB's volume will only take up 25% of the volume of water in the sump/DWC volume.
I have used 2 layers of 1/2" foam, one on the other without problems. Place the two sheets over each other and then cut to size. Keep them in that same position then mark and cut net pot holes. Moisture gets between the sheets and practically sticks them together and the net pots fit and hold them in place. Try avoiding using Glue in AP wherever.
Supplemental aeration is not necessary with the existing amount of water movement you have, although it always gives slightly faster growth with it.
I won't recommend putting fish under the raft for many reasons. They damage plants, extra layer of work to feed/clean/catch etc.
There are many other ways to style an AP system if you'd like to discuss!
You could pipe the water from GB to the back top of the sump over the existing waterline. You'll just need to make a snug hole for the pipe and probably an elbow to direct the flow downward into the sump. Although I've never used a bridge siphon i have seen them being used in aquariums, so i won't know if they are reliable, that will be up to your experience. To save cost, I've piped two tanks together using male adapters. I heat them before threading them through a slightly undersize hole, they make a watertight seal. I've also seen AP'ers use less expensive uniseal slip fittings for this type of application, so you've got some alternatives to consider. No....... no problems with roots since 1 inch of the pot is in the foam and roots tend to grow down from the second inch of the 2 inch pot submerged in the water. Maybe you could shift the sump forward in front of the GB's to get the full surface area of the IBC to be used for raft, this way you'll have to complete DWC growing even more plants.
I agree with Harold. You don't want to raise fish in your raft bed. They'll just eat the roots off of your plants. With a system your size, the circulation from your pump, along with an aerating bar or whatever else you can do to get oxygen in your tank, should be enough. You shouldn't need additional air. However, people have found that when you disturb the surface of the water by the plants roots in a DWC bed, it enables the plants to exchange nutrients with the water at a faster rate, resulting in healthier plants. So that would be something to keep in mind.
With the air surface interface of both the FT and the GB's, combined with the water movement per 24 hours with the pump you have, should allow a fair Dissolved oxygen content in the water, more than sufficient for growing your plants and fish without adding supplemental aeration. If you want to increase DO levels you can add an aquarium air pump(only 3 watts or so) in the DWC's. The return line to FT can be placed to create clockwise/anti clockwise water movement. The drain line can be placed center bottom FT, up and out to GB's.This will suck air from the surface of the FT through the entire water column to its base before going to GB's. The GB's by themselves offer great aeration to the water going to DWC's. You can also look up DIY for a simple Venturi to increase DO levels. The majority of AP systems designed as yours, produce well, and without any added aeration!
I believe that I do have sufficient DO in my system, I have implemented all of the suggestions you made in the last post in my initial system except for the supplemental air pump. My previous questions in my last post were not raised because of a concern of sufficient DO. My questions were directed at Alex's Statement
" However, people have found that when you disturb the surface of the water by the plants roots in a DWC bed, it enables the plants to exchange nutrients with the water at a faster rate, resulting in healthier plants"
So my question is how can I disturb the water on the surface of DWC bed?
Water movement transports the nutrient in the water throughout the Ap system and to the plant roots. Ap systems are generally poor in nutrient, but because of the frequency nutrient as well at oxygen is being delivered to the roots they show accelerated growth when compared to soil plants with less motility. The water movement then, will be the factor to determine nutrient transport in AP. Water movement may also be called agitation, if we want to look at it that way. From the rules of thumb(main page), we try to pump the volume of the FT in an hour, this seems optimal for plant growth in AP. If we have sufficient DO, use a good quality fish feed, and are within the rules for water movement, we have a dynamic AP! From your design you won't have any problems growing plants in the DWC.