I remember the type of handles you are talking of being fitted to the whole Flatacraft range from quite early on in production.
I also remember making some replacements with my father when some of them on our Force 4 got beyond repair.
We cut some circles of Hypalon with slots in the middle and had some short lengths of webbing sewed back to back with about an inch and a half at either end left loose.
The webbing was pushed through the slot then opened up and glued back to the hypalon circles. Bit like a split pin or old fashioned paper fastener.
With one of these on each end it was then just a case of gluing them to the tubes.
RIBBED For extra pleasure.
Member of the ebay Blue RIB cover club.
Member of the Bombard 380 Aerotec club