Be aware that if you client has more than ~50 employees, you are very unlikely to end up working under your MSA. By all means try, but be ready to have your lawyer review their MSA, because their MSA is going to be the basis for your project.
You might almost be better off trying to sneak in under the MSA with the "simplest contract that could possibly work" than you'd be by formalizing the relationship with an MSA. It is very possible to get a one-off contract executed, especially for one-off services. On the other hand, if you ignite the "MSA" neuroreceptors, you may find yourself far worse off than if you had simply gotten a Nolo-style work-for-hire contract signed; for instance, it's the MSA that's going to have IP clauses, noncompetes, insurance requirements, and things like that.
Be careful and don't wave things like "MSA"s around just because they seem like best practices. They are, for a company like ours or like Hashrockets; we're set up to negotiate and review MSA change bars. You on the other hand may not want to round trip with a lawyer several times just to do an $8000 PHP contract.