Hi @blackcat001,
Thank you for your question and welcome to the Bunnings Workshop community, it is fantastic to have you with us.
When you say the fence posts are varying sizes, are the top of the palings still in line? Or do they vary significantly?
Is it possible to get some photos of the fence? This would certainly help our members to offer their best advice based on your specific circumstances.
There are options such as this Matrix 1800 x 430mm Charcoal Slat Extension Fence which could be used, or you could build a custom fence extension using 42 x 19mm 2.7m Pre-Oiled Merbau Solid Screening to create a custom aesthetic look.
You might also like to check out Fence extension for extra privacy by @Shannon as some possible inspiration.
Let me know if you need a hand uploading images, I'm happy to assist if required. Once you've uploaded some images, I'd be happy to offer further assistance.
Jacob
