One of the vendors that is on here sells a flex vinyl welt that is made to hide/cover just such gaps. Of course I am not saying it will fill a 3/4 inch gap. But look and see.
If not maybe you can jockey the gaps to nearer 1/2 inch each.
Don't have time to look up which vendor, sorry.
The flexible base material that you mention is useful in many McGiver situations, but not sure it will work as a welt. But there are other vinyl moulding items that might work as a filler to be found at most box stores. Another fix might be using wooden screen moulding or lattice to wrap around the curved walls. I'd not get the finger jointed stuff if I were going to bend it much. If need be kerf it slightly (shallow) every inch or two to ease the bend. You'd still need a welt to sit atop the filler I think.
____ d'drummer ____