Well, 250mm is about an inch and a half too long, but I didn't need to trim it because pokes through the hard foam insert (see images). I didn't want to trim the rod because sometimes that can make it hard to screw anything on the trimmed end. It worked out just fine though. Rods of this type seem to come in widely varying sizes, like 100mm (way too short), 250mm (a bit too long but not too bad) and 1m (way, WAY too long).
You can also see the washer to keep the tire from putting too much pressure on the black ventilation enclosure.
(You can see the top of the rod poking through)
I now keep the organizer on top and it doesn't look too bad. Just for the sake of completeness, here's how the battery install worked out:
It's actually TWO of those RB20 batteries, but only one is connected. I'm using the other to fill the space since they are so much smaller than the one they are replacing (and only weigh about 5 pounds too!). Long story short: The first one they sent was fine but I had casually mentioned to them in my review that it shipped with some red paste on the positive terminal so they offered to ship a replacement but to keep the original (these batteries are not cheap, either!)
Have a good one.