You need a bikini that fits the -C shaped channel rather than the -= shaped channel?
I don't recall which models came with that style spline in the bikini, so you may be better off buying a newer CJ style -= channel. This opens up a bunch of bikini options. I know you said "without drilling your WS frame", but what about removing the existing -C frame, lay it over the new -= frame to mark the hole placement? This allows for one set of holes in your frame to work with both channels. I did this to my channel to reuse the nutserts in the frame for the hardtop
Back to the bikinis... Do you have a rollbar of some sort? If not, what are you gonna use to hold up the backend?
I used to run a CJ7 bikini, but it was too long (hung down too far in the back & blocked sight of rearview) and I've since gone to a YJ "strapless". If you decide any of the above channel options won't work and you must use the -C channel, you may be able to use a CJ5 or 7 bikini (plenty of length) and have the forward edge modified to use the round spline (most come with a flat spline for the -= channel)
HTH