Well, just some more opinions and some thoughts:
I believe it is possible to "ammend" your present will to keep the attorney costs down. Take your copy of your current will, make another copy, cross-out the parts re blood, add the part as to how you want your eventual funeral conducted. Do this - before - you consult with an attorney. It will save time and money.
Also, see a planning consultant at an established funeral home - there is usually no charge for this service - inquire about prepaid arrangements, and re what you explicitly want done (re services, burial,etc.) upon your death. These consultants are quite knowledgeable - I wouldn't be surprised to learn he/she have dealt with other situations similar to yours. For example, I personally knew a Roman Catholic couple, she was quite devout, he couldn't have cared a hairy rat's bum. In fact, he carried a notarized card in his wallet, saying he did not want a Roman Catholic mass, burial, and didn't want 'last rites.' Additionally he had his hospital records marked with "no last rites." He had prepaid and prearrangened his funeral.
His will also reflected this, but in order to take every measure he could to insure that his wife didn't alter his wishes, he tried to cover all bases.
He also entrusted and authorized a good friend (who also was pragmatic and didn't want a religious funeral) - to immediately, upon learning of his death, contact the funeral director to see if his wishes were being carried out. Morticians are licensed and legally bound to carry out pre-arrangements. This, did indeed, stymy his wife. She wasn't about to pay-out additional $$$$$ for the services of another mortician. So, he got what he wanted in terms of final arragements.
The financial picture may be another story. I know that there are 'trusts' that will provide the survivor with a regular, scheduled income, and that also allow said party to receive 'extra' for valid purchases and/or wants. They can also "exclude/deny" such things as large donations to evangelical associations, yes, even the WTBTS. However, they may have loopholes which your spouse could utilize if she was gung-ho re giving the society a considerable sum -- for example, she could 'want' a diamond bracelet, then turn around and present that as a gift to Brooklyn! (You know they have advertized/advised on such things.) If you have no children?? to pass a portion of your estate on to, you can also specify that upon her death the remainder of the estate will go to some worthwhile charity, etc.
The financial issues are where it will cost you - you'll need an estate attorney/executor for such arrangements.
Probably well worth it in order to see your wishes carried about re the organization not benefiting are honored.
Again, these are just my thoughts. So, start with some basic footwork. If you have a university law school near you, that would also be a good place to check out some estate planning texts for ideas. And, sometimes you just get lucky and find a 3rd-year law student who can be quite helpful in your search - hey you might wind up being his first client - once he passes the 'bar.'
And, it's no crime to invite him to a bar for a round or two, either. :) :)
Just my 2 (hmmmm, I've heard that somewhere, hehe.)