Unfortunately a will is no guarantee you won't be cremated.
The funeral home will honor the famiily's wishes expecially if your funeral is not pre arranged and pre paid for. And once you're dead there's nothing you can do about it.
Also, wills often aren't read till after the funeral.
The best you can do is make your heir the executor and inform him/her ahead of time that the will states that if your wishes (in writing) are not followed he/she will NOT receive anyghing and your estate will go elsewhere.
Money usually talks.