Basically, do everything PearlDrums just said.
"I'll wing it by using the dimensions I've found on other sites describing the dimensions of fiberglass HH bows." It's hard to exactly copy fiberglass bows, but what you are looking for is very similar tiller and finished unstrung profile.
I might have suggested 1.25" wide, just because wood is wood, not F/G, but ipe is indeed fantastic wood for this, being both quite stiff and pretty darn elastic, so as mentioned, it doesn't take much to make a bow as far as thickness, etc.
I also have a long draw, which is a source of occasional frustration when harvesting staves. There are SO many that are perfect otherwise, but not QUITE long enough for 29" plus, or the higher draw weights, or whatever else. But, you either add width or thickness, and in this case, more length probably.
But, 72" is pretty long already, should be ok.