The secret project is now finished; I'll post a photo later this month once the recipient has received it.
I have finally started to dip my toe into illusion knitting. After a couple of false starts I decided on Ysolda's skull illusion scarf. I've slightly altered the pattern - added two extra garter stitch rows at the start and an extra three garter stitches at the start and end of each row; this stops the scarf curling at the sides and ends; it also makes it a bit wider which I prefer. Assuming it turns out as planned and I can finish it within a month, it will be another birthday present.