I made my first mockup. I took the pattern off of my stays from Period Corsets. As Dawn Luckham pointed out to me, the gussets on these are much longer and deeper than was common for early 1800s stays. They're more appropriate for 1830s really. So I will have to change that.
Also, I experimented with a couple different busk options.
From left to right: a plain piece of wood, a wooden ruler, a wooden busk from Hedgehog Handworks, a straight wooden ebony busk from Lacis, a tapered wooden busk from Lacis. The tapered busk is more appropriate for 1700s corsets, but if I turn it so that the wide part is down, it sort of holds in my stomach better. I decided to go with the straight wooden busk from Lacis.
The Lacis busk (on the right) is much thinner than the Hedgehog Handworks busk, so it has more give. I like that in my regency stays. Much more comfortable. Although I use the Hedgehog Handworks busk in my 1820s stays, because I made them for a thicker busk.