Actually if each of the villas are on a separate (privately owned) Title then it sounds like you’re dealing with a “dividing fences” matter. (between each villa and the Common Property). Same as if you were dealing with a fence between the Common Property and an unrelated neighbouring property.
If so then the general guiding principle is that the cost of any surveying, and construction of any dividing fence/structure would be shared 50-50 between the two parties.
Getting agreement ahead of time is “better” but not completely necessary. When agreement can’t be reached usually one party (the OC in this case?) could construct a not-extravagant “fence” and send the other party an invoice for 50% of costs. If the OC chooses the quote/contractor this would also help ensure the “fence” structure was of consistent appearance and construction-quality across the whole property.
Most states’ laws will fully support 50% of a “reasonable” fence structure as an enforceable debt, regardless of whether the other party has agreed or not. (but it’s better if they do agree)
In the very worst case the debt is recoverable against the proceeds of sale of the other property. Also depending how the OC issued or structured the debt to each villa owner you might find that a non-paying villa owner becomes “non-financial” and unable to vote on matters at OC meetings.
Would suggest dividing total costs by length of all “fencing” constructed (= $/m figure). This makes calculation of costs to each villa straight-forward. (50% of $/m by the length of fencing for that villa)
I don’t see why this would not also apply to lesser “fencing” if you did not actually want a fence constructed for some reason. Such as rows of timber bollards, such as you see around recreation areas. Or any other infrastructure used to delineate the property boundary.