Water shields are depicted on page 11 of the 66 Mustang Interior Assembly Manual. The pictures I took when I reworked the vents on my 66K show the water shield folded down and are not what you need, but the assembly manual is, in this instance, accurate. There's a combination of strip calk and tape used. "Gaffer's tape" is what most people use as the sheen is very similar to the original tape. The only drawback to "Gaffer's tape" is the adhesive side is white while the original tape Ford used adhesive side was the same as the exposed side.
Again, I strongly recommend that you get a set of Assembly Manuals for your cat.
Jim