What size are the holes in the frame and what size are the holes in the rack you want to mount? You can use bolts with thin hex heads, mounted with the head on the inside, or you can thread the frame. 5mm is the size I most commonly encounter, but if the frame holes are bigger, you may be able to thread them for 6mm, which is another common size. If all else fails, and you don’t mind burning the paint, you can get a couple of threaded flanged mounts that are used for brazed in water bottle mounts and drill the eyelets for them and braze them in place. Silver has a lower melting temp than brass and can be used to avoid damage to the brazed joints of the dropouts.