Thank you Scoot, I already noticed some vendors sell those cranks on ebay, but only a few of them offer the right ones (i.e. the clip-on style) with the appropriate insert.
Moreover, I would have liked to keep my original handle if possible as only the knob is damaged.
When inspecting the faulty knob, it appears that the black plastic has worn out , probably because of "off-axis" stress as my windows can sometimes be hard to wind up when soft top in on.
As a result, the opening is too wide and the knob gets loose on the crank fitting (the translucent plastic part).
So, after various attempts with neoprene glue and trying to fit an o-ring clip from the inside, I eventually used a heat gun to soften and then re-mold the black plastic, trying to get back as close as possible to the original shape and diameter of the original opening.
Well, result is not too bad; the knob doesn't disengage anymore when cranking!
Anyway, i feel time for replacement might be coming sooner or later, and I'd better order some new "crank + insert combo" just in case...