I have the same problem as I'm the sole usability engineer in the company and the rest are Java developers. They feel threatened because they think they have to code unnecessary or "fancy" functionalities.
Gather user data and present it
To convince developers that the proposed features are valuable you should present user research data. The key is to use data to convince them your suggestions are not based on personal opinion. It will be great if you can present your findings frequently: every week or twice a week. This way programmers will be acquainted with the research. Hopefully, they'll take into account some user problems.
Programmers can be convinced a feature is worthy of developing by presenting them:
videos of user tests - showing them users struggling with the interface is maybe one of the best methods of influencing developers. This approach worked well for me.
statistics - presenting objective data, like time to complete task, should work well as tech engineers work a lot with numbers. Use different charts and visualization for higher comprehension
comparisons before and now - by comparing efficiency of new components vs old, or new interface vs old is great. They can see the gains that could be achieved.
academic refences - if you can show that your design is validated by previous academic research. This way you show tou are not the one that invented it worked really well for me.
Be positive and prepared
Programmers will often try to deny development of functionalities which will take a lot of effort. You should expect that and be prepared with arguments. However, try to sound positive and sustain a healthy dispute. Don't lean towards negative emotions. Listen developers, respect their opinions and gently tell them your view. Your communication ability and presentation skills will determine the success of your endeavor.
Create a user-centric culture, if possible
If you want to have long-term adoption of User centered design you should create a culture of testing with users. This is surely the best thing to do for the long-term success of that company.
However, I don't see how you could create user-centric culture when you are only a consultant for a couple of months. This is a long process and it takes many years.