I had already been thinking I might need a sheer fabric for interfacing the top collar. So a solution popped into my mind: an organdy undercollar! This way the undercollar would provide support and transparency.
I bought 1/2 yard of very sheer cotton organdy to cut the under collars out of. I looked for something relatively stiff, that could stand in for the role of interfacing. Silk organza would work well too. I just sewed the collar as usual (see my tutorial here) and trimmed the seam allowances down to a scant 1/8". When pressing, I made sure that the organdy was rolled to the underside of the collar.
Here you can see the technique in lace, in progress.
Anyway! Just a little tidbit to keep in mind any time you need sheer support in a garment.
Peter Pan collars FOREVER!!!!