[Lauren_]

Simplest thing would be to start with a lingerie bra that fits and cut away all the straps so you're left with just the two cups, still attached. Then you can replace the straps around the back with something non-stretch and add shoulder straps. Depending on the fabric of the bra you're using, you may or may not want to cover the cups as well.

For the back of a bra, I cut the shapes I want from heavy fusible interfacing. Then I fuse it to my fabric, then cut, leaving a small hem allowance. Clip curves & corners, turn under & stitch in place, then attach to the sides of the bra.

Shoulder straps are done in a similar way... sometimes with the same fusible interfacing, sometimes with ribbon.