My experience from building the overnight sensations (both tm and mtm), you need very little stuffing. The insulation you have is fine. Just spray some adhesive against the back wall and put a full (compressed) handful in there. Wear gloves. You don't need to line all the walls, just the back of the woofer. The amount needed wouldn't cover all the walls anyway. Anything will work though. I personally use stuffing from an old pillow as I don't like insulation.
Always wear a mask.
If you are trying to do this in your kitchen, I recommend sanding edges till smooth and apply a sealant or acrylic primer.
Smooth over with spackling compound (heavy patch) if needed. Apply another coat of sealer.
If you have to use a brush or roller for the painting, moisten the brush or roller first and blot dry.
Highly recommend a textured paint if you've never done this before. Getting smooth finish can be done, but it takes lots of practice and sanding.
I use a block sander, and then sand using my hand around the edges. That way you can feel any imperfections as you are sanding.
Take your time and don't worry. You can always sand mistakes down.
For the crossover just use velcro. The kind you find with the sticky tape on both sides. Cut a small square of the that for each corner of the crossover board.
Place the crossover on the bottom of the speaker.