Carpenter/ joiner
The ABTT says:
The set will be built in the scenic workshop by the master carpenter with the help of one or more assistant scenic carpenters. The master carpenter is most likely to have acquired the skills through starting in the department as a trainee or at drama school, although it is possible that carpentry skills learnt outside the theatre can be adapted to suit theatre practice. The same applies to scenic carpenters. There is a skills shortage in this area and good construction professionals are in demand.
In addition you might find that a welding qualification could help you with jobs during get-ins, fit-ups, etc
