I have done some great T-shirt alterations but bra straps showing in the front and/or back is an issue. I am too well endowed to go without, and a strapless bra will still show in the ones with cut out backs. This will give you a little lift, and will look more like a tube top from the back (check out the pic with the blue shirt). *I wore it around today with success. Nothing fell off or out even when I leaned over or bent down to pick things up. It requires adjustment, but no more than any other strapless thing I have ever owned. I wouldn't play volleyball or chase small children all day in it but I wouldn't do those things in any strapless item ;-) *
Start with a rectangle of stretchy fabric - I cut up an old peasant top. You want it to be long enough to wrap around the front and tie. My bustline is about 45.5" and the finished piece was 60" long. Mine was 14.5" wide. I think wider is better for support and also looks more like a top from the back.
Mark where a seam would normally be with a safety pin. This is because we will taper the front and not the back. (sorry for the bad pic)
At the edge of the fabric mark a point in the center. On side draw a line from that point to the safety pin. Do the same for the other side. This is the front triangle.
Cut out the triangle. It should look like this. Repeat on the other side.
Wrap around front, tie, and that's it!