How to install rear rack & fenders on a vintage road bike
Does this frame have mounting points down on the rear dropouts?  The sort  sometimes used for mudguards/fenders, which can be repurposed for the downshafts of a rack.  Since they hold most of the weight, a front mount is relatively lightly loaded.

I made a couple of clamps out of thin sheet metal for my rack, which wrap around the seat stays and bolt through the rack.

View from above
 

Looking backwards


Each bracket is a short strip of aluminium which is wrapped around the seat stay and then back on itself.  The bolt is a stainless steel one, and is (edit: should be!) double-nutted for vibration resistance.

Remember at least 2/3 of the weight is taken by the rear struts.  I've had a good 12-15 kilograms on this rack.   The black plastic is a substitute for a mudguard/fender, which does fine protecting my backside and bag, but does little to keep the bike and chain clean.