Two factors will stretch you - firstly the Boater crown is a low crown 6cm and capelines always have good height in shape of crown so you have excess height and the next challenge is the brim is perfectly flat so stretching that out all at same time will need time, muscle & patience!! Even if you were to use a flare parasisal which will make crown easier but flat brim will be difficult. Felts are successful all in one but not for Spring.
Sinamay boater has to be made in 2 pieces but you can make a boater all in one using Swiss braids as in this link. This is my choice as a traditional look for a Boater. I had a milliner recently say to me I must buy a boater block -my reply "Turn a brim block upside down, drill shallow holes to position collar with steel pegs and you are ready to go" Tip = Don't have brim too wide! Definitely include a boater in Collection this Spring with a touch of blue in keeping with the popular marine theme.
More info: http://hatacademy.com/group/l40