I drive an old car with simple mechanical door locks.
One of the door locks has broken.
I might replace the door lock, but I would like to ask if anyone has a suggestion regarding how to secure the door from the outside in the mean time - Any DIY tips?
Depends whether you're bothered about damaging the outside of the car. Weld it shut, or fit a hasp & staple with padlock. Or fit a domestic door bolt on the inside & operate the bolt before you get out of the car by another door.
I would think a six foot length of 2 x 2 would be enough to span the door and reach half way along each of the adjoining panels - fix that on with a tube of no-nails and the job's a good'un.