This is an interesting project. You have good ideas. Last year I made a down-filled cuben sleeping bag with integral pad, so I thought I might contribute a couple of things I learned.
Cuben is UHMWPE fibers between layers of polyester film. It doesn't heat seal well, in my experience, because the fibers shrink and deform the material at a much lower temperature than that required to seal the polyester film. Also, although cuben is absolutely airtight and has good tensile strength, the resistance to puncture is poor. Thorns that can't get through 0.75 oz nylon will puncture the lightest cuben pretty readily.
Also, here in Oregon the relative humidity is often high, and I found that nighttime cooling would cause dew to condense and wet the down in the outer layers of my sleeping bag. Because the shell (inner and outer) was cuben, it was very hard to dry out. It just had a small valve, like an inflatable pad. New dew just formed each night, and the down got wetter and wetter. I don't know if the same might happen in a DAM.
The Cuben fiber staff recommends a Loctite two-part urethane adhesive called Hysol U-09fl for bonding cuben. I actually used the low viscosity version, U-09lv, and found that an extremely thin film of the adhesive produced a lightweight bond stronger than the cuben itself. Tape is simpler, but much heavier, and I've heard anecdotes about tape failure in very cold weather.