Commercial Pattern Fill-in #2
This fix is very similar to Fill-in #1, but slightly more complex because of the shoulder seam and set-in sleeve. You will need the commercial pattern New Look 6379 (note: I purchased this pattern at WalMart in the fall of 2006):

You will use the pattern pieces for the front, back and sleeve in the white dress view to create a filler insert for the spaghetti strap/strapless dress. Open the pattern and find the instruction sheet. It lists the pattern pieces. You will be looking for pattern pieces 5, 6, and 9:

Find these pattern pieces and cut them out. Pin the front and back pattern pieces together at the shoulder, then put your dress on and drape the filler piece over your shoulders to see if it gives you the coverage you need. Adjust as needed (i.e., bring the neckline up, bring the shoulder in, add length, width, etc. You may wish to overlap the front pieces at center front to bring the neckline up high enough). Fit the filler piece to the dress, adjusting as needed.
Once the pattern pieces work, use them to cut out your material. If you wish to use a lining, cut that out too. Stitch front and back at shoulder. Hem sleeve and attach to front/back. Hem, face, or line neckline and center back. Stitch the side seam. Then, while you are wearing the dress, put the filler on, fit it to the dress, and and hand-stitch it in place at the bodice front and back. Attach a button and a loop at the top, or a hook and eye, to close the filler piece at top back.