I extended the rear rack I had by popping off the aft-most plastic part, and fitting some scrap metal tubes of a suitable ID. Worked well enough, will get a photo soon. TBC

One of the problems with widening a rack is that you may suffer from thigh strike if the platform is wide enough too soon.

Another option is to fit a front rack to your bike as well, and carry loads there where you can see them.

I extended a rear rack I had by popping off the aft-most plastic part, and fitting some scrap metal tubes of a suitable ID. Worked well enough, photos below. I also "decked" the carrier with some tough plastic, forming a serviceable mudguard up to about the seat stays.

One problem with widening a rack is that you may suffer from thigh strike if the platform is wide enough too soon.

Lengthening a rack can put any "clamp" mechanism in an oddly "forward" position, as pictured. Some modern racks have fancy proprietary clamp/rail systems which may get in the way or not work.

Another option is to fit a front rack/basket to your bike as well, and carry loads there where you can see them.