It is generally not a good idea to rely on your will for expressing your preferences for funeral arrangements, because the will may not be found or read until several days or weeks after your funeral.  The better approach is to write down your wishes in a signed and witnessed document.  Give the document to your spouse, children, trusted friends, and (if you know who you will use) your funeral director.  You could name a "funeral agent" in this document, someone you entrust with carrying out your wishes.  It also helps to talk to your friends and family about your plans and ask them to cooperate.
 
The reason I suggest giving the document to the funeral director is that, under Massachusetts regulations, if you have signed a pre-need funeral contract, or even if you have merely given a signed witnessed document to the funeral director, he or she is bound to try to honor your wishes.  See 239 CMR 3.09:
 
Control Over Arrangements and Disposition of Human Remains
 
Every registered embalmer, registered funeral director, and licensed funeral establishment shall comply with the following rules with respect to control over funeral arrangements and disposition of human remains:

(1) If a licensed funeral establishment is a party to a pre-need funeral services contract, as defined in 239 CMR 4.01, for the benefit of the deceased person, and said contract is still in effect at the time of that person's death, the terms of that contract shall control the nature of the funeral goods and services to be provided, the manner in which funeral services are to be conducted for the deceased, and the final disposition of the deceased person's remains to the full extent provided in that contract. No licensed funeral establishment, nor any of its agents or employees, may cancel or materially alter any of the arrangements specified in that contract, even if requested to do so by a member of the deceased person's family or any other person, unless:

(a) Compliance with the terms of the original contract would result in a violation of any applicable federal, state or local law or regulation; or
(b) The funeral establishment is ordered to do so by a court of competent jurisdiction.

(2) Where there is no pre-need funeral services contract with the licensed funeral establishment in effect at the time of death for the benefit of the deceased person, or to the extent that any such contract does not fully specify the nature of the funeral arrangements to be provided, said funeral establishment and its agents or employees shall give effect first to any wishes of the deceased person regarding the nature of the funeral goods and services to be provided, the manner in which funeral services are to be conducted, and/or the final disposition of the deceased person's remains, which have been expressed in any written document which was signed by the deceased person in the presence of a witness. In the absence of an order from a court of competent jurisdiction to the contrary, said written document shall be sufficient legal authorization for implementation of the arrangements specified therein.
 
Finally, depending on how 'unusual' your funeral preferences are and on how strongly you feel about them, you should discuss this issue with your estate planning attorney and ask him or her to help you out.  It is possible, for example, to include language in your will that disinherits any person who opposes your funeral plan.  If it were me, I would include similar language in the document described above (the one you will distribute to family and friends), so they understand how important it is to you and that there may be consequences if they oppose your plan.  Good luck.