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Very nicely done! I think you could substitute any kind of closed cell foam, such as sheets of pipe insulation or even cheap sleeping bag pads used by hikers. I agree with others who mentioned that gaffer's tape would probably look more "professional", but it's relatively hard to find compared to duct tape, so I have no issue with using the grey stuff. Adhesive backed hook and loop fastener would also work to keep the dividers in place.
I think you missed the humor in Mark's comment?
Easy Configurable Cargo Blocks for Your Car Trunk