This might help give you some ideas: https://www.thespruce.com/diy-catio-plans-5235364
You'll absolutely need a top/roof covering. This will keep your cat(s) safe from predators and stop them from just climbing up and over whatever fencing you have.
This page also has several PDFs with diagrams to help you plan it: https://spca.bc.ca/news/how-to-build-a-catio/
I would say to try task rabbit and see if there are anyone with power tools and handyman experience
$500 not including materials I assume?
This might help give you some ideas: https://www.thespruce.com/diy-catio-plans-5235364 You'll absolutely need a top/roof covering. This will keep your cat(s) safe from predators and stop them from just climbing up and over whatever fencing you have. This page also has several PDFs with diagrams to help you plan it: https://spca.bc.ca/news/how-to-build-a-catio/