I see you are in Portugal so I can't really help, but over here any chandler would sell you some repair tape, or a sailmaker would give/sell you a piece of suitable stuff.
I think spinnaker repair tape would be on the light side for a try-sail. If the self adhesive stuff is not available in Portugal, how about contact adhesive, or even needle & thread.
You need self-adhesive insignia cloth. Heavier than spinnaker tape and available in larger patches. I'm sure any sailmaker,..... or I've used this place: