The best response I've ever heard of was from someone who was being badgered into volunteering for some thing (I forget which). The response was this:
That is certainly a valid concern and worthy of all your time and energy. Unfortunately, it is not my concern."
But I suspect that uncle won't accept that. Some people say to say "no" with a reason behind it, such as "I already have charities I donate to." All that does, in my experience, is give the pursuer an argument to pursue. "That may be, but this is more important. Donate to my charity instead." Or "That's good, but all you have to do is skip this spurious activity and you can fund my charity."
I've been the "victim" of this in the past, and it relies on your guilt to make their goal. And the pursuer knows that it's worked on others in the past, and they're going to apply pressure to you now. My response to that is, "No, I'm not in a position to help this one out, sorry! Good luck with the fundraising!"
And offer NO other information. You don't owe an explanation or excuse, despite how they pursue one.
When the inevitable guilt comes, and some people like to lay it on thick, refuse to accept it. "You don't want our church to help..." "That's not what I said, and you know it. I'm not accepting this guilt so please stop now." Be firm and stay on message. And after a sentence or two, just walk away. The best interpersonal skill is to NOT engage in foolish arguments - especially with family. All that accomplishes is to get you a bunch of calls from well-meaning but annoying relatives.
Keeping in line with this, I've seen too many arguments start because someone didn't stop there. "I'll send you the link." "Fine, do that" and quit talking about it. You can be the bigger person, let them have the last word, and delete an e-mail. Otherwise, too many arguments start with "I'll send you the link" "don't bother; I won't read it anyway" and I've seen that devolve into another argument.