I don't think I've posted this before.
As I'm cleaning out my observatory I found my old Bahtinov Racquets. They are used just like a Bahtinoc Mask as a focus aid but the racquets obscure much less light than a mask, so they work well on dimmer stars.
I made these from 3mm MDF and so they are not very robust (especially after 10 years or so of knocking around the obs) but when you work in an MDF factory and you're building something experimental...
If you want to make some that'll last I'd recommend some thin plywood (<-3mm) or perspex or acrylic sheet. Use fishing line for the strings.
I've attached the full scale patterns for the hole drillings for racquets for a C11, an ED80 and a camera lens for anyone that wants to make their own.