I saw this pop up a few days ago on eBay and was super excited to see it get done, but wanted to wait a while in case Dimmog was going to post this himself.
Months back when I was emailing back and forth with Keysight about getting a replacement rear bezel I remember messaging Dimmog asking if this was a project he might tackle since he already makes 3D printed rear bezels for the 34401A, but back then the answer was no as his existing printers didn't have print beds large enough for the 3458A bezel.
Looks like things have changed recently and I'm super happy to see we now have access to this:
https://www.ebay.com/itm/327143850863Way more reasonable price than buying a replacement from Keysight, if they have it available - last time I eventually got an accurate quote from them it was $212 + shipping. Dimmog had 10 to start with and already sold out, but I've talked with with him over email and he's printing more.
EDIT: I forgot to add this when I created the post - he has his own site:
https://hubequip.net - you can order directly from him there instead of going through eBay.
EDIT 2: direct link to the bezel on hubequip (Thanks Josh!):
https://hubequip.net/products/hp-agilent-3458a-rear-panel-plastic-bezel-also-fits-3245a-3d-printed