I'd say make a bendy or slight bendy first and see how that particular wood/tree responds,and how much set it takes,and the outcome then go from there to see if it can take the strain of a stiff handle at that draw length. You shouldn't have to go any wider than an 1 1/2" starting width at the fades with rhamnus cathartica with a 63" bendy flatbow ,and it'll probably end up being narrower than that for sure like closer to 1 1/4" or less. And narrow your handle section a lil less than your starting width.
Btw...where are you indoors with drinking fountains on the wall with staves??? A school?..lol