It turns out that the couple of millimeters extra length of the new battery holders for the FT-290Rmk1 actually do matter! They didnt fit the carrier without bending it, which of course is out of the question, because then the carrier wouldnt fit back in the radio.
So I trawled the online suppliers looking for replacement replacement battery holders, two millimeters shorter. The only ones I could find were twice the price of the ones I had, and didnt look like they would survive a stiff breeze!
Faced with the possibility of a long protracted and expensive hunt for suitable battery holders, or even less likely - Yaesu originals! - I started to reason the problem out, like this...
The PLASTIC battery holders fit into a STEEL carrier.
The TERMINALS are at either end
The PLASTIC in the middle only keeps the batteries off the carrier
In other words - the main body of the holders is just there as a guide and insulation for the batteries, it contains no electrical parts, and when installed in the carrier, has no mechanical value either. So, since the carrier provides all the mechanical support, whats to stop me simply cutting the holders in half, shaving off the extra length from the middle, and putting them back together within the carrier?
This is exactly what Ive done -
Clamping the holder in the workmate, I brutally amputated one end with a ripsaw. This took about a millimeter out, then, with the aid of side-cutters, I shaved another millimeter off one of the halves.
One relocated into the carrier frame, the cut is irrelevant. A little polystyrene Q-dope into the gaps and they are once again complete holders.
I also took the opportunity at the same time to replace the aging insulation tape between the carrier and the regulator PCB, with modern cloth based insulating tape.
I have yet to test the new holders, as im waiting until the dope is completely cured.
UPDATE - Carrier refitted and secured, NiMH 4000mAhr C cells installed, and lid replaced. Radio powering up and working on internal batteries. Job Jobbed!