Brim. I added 8 extra stitches for a brim.
I opted to use the left side for the brim, instead of the right, since that chain-stitch edge seemed tighter. It meant breaking the yarn and starting the top at the bottom instead of the top, but the nicer edge was worth the extra ends.

Circumference. I only needed 68 rows for a 19-inch circumference (3-inches negative ease). So the top tab was 17 stitches by 17 rows, or a quarter the number of rows (which needed to be divisible by 4).

Seaming. I used the seaming method described by 10rowsaday for slip-stitch edges in knitting. It was perfect to join the chain-stitched edges. I adjusted the seaming on the first 8 stitches of the side seam so that the nicer seam would end up on the outside when the brim was turned up. Had I continued to use the inside half of the chains all the way through, it might have produced a seam which better matched the Tunisian crochet.

I sewed the side seam first. I placed markers every 17 stitches along the top, pinning each side when I was ready to sew it. Next time I may try to incorporate the seam as I work each row of the top, and, for that matter, try joining the bind-off edge with the foundation row at the bind-off.