It seems that silk linings for flat caps go pretty far down the social scale, so I dug out some scraps from my Extremely Purple Gamurra to use. (This is not really an appropriate period choice, as the silk is a slubby dupione and the purple is way too optically bright, but I don't have any more appropriate scraps and I'm not cutting into virgin silk for a first-try project. Also, S. likes purple.) I cut out the matching lining pieces and started assembling. The Tudor Tailor pattern is pretty straightforward and by the end of that day I had the crown basted and gathered and the hatband & lining attached on their appropriate sides.
The next step is to attach the brim, but I wasn't feeling happy about it. The wool is a very light, loose twill, and the silk is stiff for its weight but is also very light, and it all felt like it was going to be super-floppy even by flat cap standards. I thought it best to reinforce that a little, so I dug out more scraps--dark brown cotton canvas that was part of the interlining for my 16th c. gown--and interlined the brim.
Protip: Do not try and retro-fit this into a brim & lining you have already sewn together. Just fucking unpick it and do it right. It will take a fraction of the time. ASK ME HOW I KNOW.
Finally all the hurdles were overcome and everything was attached and I had something that, by gum, rather looked like a flat cap! I did my victory lap around the living room and then had the recipient try it on--whereupon it perched on top of his head like an oversized fascinator. Somehow, in spite of careful measurements, it came out about 2.5" too small for his head. This is far too typical in my life.
I put the thing aside in disgust for about a week and dove into some pourpoint research (more about which later) before finally gritting my teeth and unpicking the pieces to see what had gone wrong. Upon extracting the hatband and ironing it out, I measured it and it actually is the right size for his head. I guess what's happening here is that the extra volume caused by the gathers turns the perfect-sized hatband into a too-small one. I think I don't have any choice but to cut a larger hatband & lining; I hope I don't have to cut out a new brim as well.
I had even more antipathy for the whole thing after this realization so I put it to the side again, and I've just been knitting for the past week (at least the damn sock is nearly done; just in time for summer, as anticipated). My attention's also been sucked away by Pennsic work (I'm the land agent/general factotum for a camp of about 70 people) and for an event I'm helping run next month where I volunteered to teach yet another class I haven't actually prepared for. And I'm about to go on vacation, which is full of yay!, but I'm feeling time slipping away and I'm not feeling good about that at all.