If there is $$ in the budget, I wouldn't reinvent the wheel as there are research-based assessments available. Most often though, the $$ isn't there, or needs to be utilized in another way. Writing your own skills assessment is do-able, and I would only caution that you take great care in the writing of them, and double, double check everything. Then have another set of eyes (with a functional brain) look at them, then check them again.
One thing you want to be careful to avoid is compounding skills into one factor. As a rather simplistic example, "Listens carefully and communicates clearly."
Might sound great, but listening and communicating are two disparate skills. I may write and speak well, but don't listen worth a darn. How then could this factor be accurately rated?
So to break this into two factors( "Listens carefully" "Communicates clearly") allows the rater(s) to more honestly assess.
Additionally, define the ambiguous to level the playing field. Using the "Communicates clearly" as an example...I once worked with a person who had a wonderful command of the English language. I understood each word that he said, but when he strung them together in a sentence, it was impossible to understand what he meant! So "Communicates clearly" could be a skill heading, followed by a definition of the entirety of the skill desired.
Has the ability to create understanding in both written and oral communications. Understands and uses correct grammar. Can spell accurately.
I hope this helps, and I wish you the best,