That bikes should be able to take full, bolt-on fenders.
Does the seatstay bridge have a bolt hole?
The rear chainstay bride is very high. If you want to extend the fender below, you may have to make some kind of bracket to the seat-tube. You can use wood, plastic, metal, whatever you can work best.
Plastic fenders can be fixed with small zip-ties, you drill a hole, 4-5mm diam at least 1cm from the edge.

Racks with top-plates make poor fenders and not very good racks. Open top framework designs allow you to lash oversized objects to the top more easily.
I have a number of rack platform extenders, bits of wood or stuff that I fix to the framework to carry bulky items such as bags of cement. I use bits of wire to provide a temp attachment and its much easier with an open framework design.