Buy a Tubus set of clamps for mounting racks on frames without braze ons. If Tubus sell them they will be up to the job. At that price buy a spare set for your round the world tour. They have the other advantage over a random P clip from the hardware store of coming in sizes appropriate to various tube diameters. Measure before ordering.
This may be a UK only site but the item will be sold in the USA also.
https://www.totalcycling.com/en/gb/T...t/m-24130.aspx