To be done correctly, this is actually a much more complex undertaking than your question implies. That being said, what you are suggesting is a fee for services contract. If the scope of services are limited you could create it yourself, although there are many issues that must be taken into consideration.
Just off of the top of my head, to name a few issues that must be considered, are: Scope of services; Definitions (important in that ambiguity is typically construed against the drafter); terms and conditions; indemnification; severability (savings clause); specific performance and penalties for failure; whether or not time is of the essence; payment methods, and ceilings that trigger another explicit affirmation in writing.
I know you'd like to avoid earning/payment caps, but contracts of this type are typically negotiated and there is usually some limit that, when reached, a change order or addendum must be added. This could include reasonable and necessary expenses on your end (again: how these are defined will be important). Very few business will enter into a contract for services with no ceiling to what they could end up owing. Contracts, in general, should not typically be one-sided agreements that benefit only the drafter as you want to ensure general enforceability.
There are, in just a very general sense, many provisions that may be necessary to create an enforceable contract depending on exactly what type of work you will be doing, whether or not you will sub any of the work out, whether your work (if inadequate or negligent in some way) can materially adversely impact the business and if so, a clear limitation of damages (if any) you could be exposed to. Also it can really matter what jurisdiction you are in.
If it is something very simple, like "John Doe will sort Jane Doe's email folder remotely once per week," you could probably get away with doing it yourself with online templates. Not something I would suggest if your are doing any type of meaningful service(s) and entering into any lucrative/long-term contract. There are all sorts of simple contract principals a layperson just does not have an understanding of, that may and often do, end up being essential. If it is something relatively simple, a lawyer could draft such an instrument for you without you needing to spend much money on the front end, and you may save yourself a lot of headaches and money at the backend.