A pacifier or bladder would be you best bet.
The pacifier pod will not fit smoothly into the wing, but will protrude above and below it. This rig has won many national and world championships, despite the apparent issue of drag.
The surgical tubing bladder fits into the model better and will look more streamlined. However, the very high pressure makes this system tricky to handle and even top level competators often favor the lower pressure of a pacifier, despite the big bulge.