Best way to either require or get TIME contingency in a schedule?   I mainly know the cons...

There will be delays on a big project, it seems like you are setting yourself up to pay more:  some thoughts?  I was on a job once where they accidently have 4-5 months too long.  You never had to worry about time claims and the contractor still wanted to get out as fast as they could to avoid overhead etc costs.  I guess you could get a contractor that would use it for fill in work and drag it all the way out - longer than needs to be.

If you don't need it for 16 months, require the Completion in 14 months, but budget in paying for contractor Overhead for the extra two months.  If you make that part of general contingency someone will snatch it for something else.

Other methods that I think are a problem.
.1. Two month reserve at the end.  No, that will mess up your floats everytime you release some reserve, you need limited new dates.  Plus that reserve effects construction activities but should not for weather - but a reserve at the end will add float to submittal items etc that should not have any.

.2.  Require 2 months float.  People dislike a schedule that has 60 days float critical paths.  People are used to seeing or wanting the critical path at 0.  If it were for just schedulers that is one thing, but it never is.

.3.  Have a regular substantial completion date, but a final completion date with 2 extra months it, then award SC early with some of the items sliding to Final Completion?

.4. Your idea: