Sorry. Something just isn't right here. On many levels.
The first being the very high likelihood that the OP is abusing the independent contractor status of the people who are his employees. Just because they don't have a set schedule doesn't make them contractors. There are checklists readily available from the IRS that will verify this. Things like giving them specialized training and providing them with their work tools are red flags. Not to mention the fact that these folks have been working exclusively all week, every week, for years on end for just one employer.
If they were really contractors, you'd simply go out and find someone else who has free time that week. There should be no great concerns about paying those outside contractors more than you're paying your "regular" guys, because you'd be paying market rates.
And it cuts to the core of the original problem. With real employees, you have attendance rules, you have vacation time and sick time and personal time. Anyone violating the rules is subject to disciplinary action or termination. You can't have such rules for contractors, so it's impossible to hold contractors to the same expectations.
It must pose other scheduling problems. If a contractor says "I'm taking two months off to tour Europe", the best you can do is find another contractor. You can't say "you don't have two months of vacation coming."