There's always the tried and true Raffle with proceeds donated to the Rangers. Small potatoes for sure but ask SloHike and RockDawg69 about it. As for larger dollar amounts - perhaps join up with the already organized groups for larger fundraising events and volunteer perhaps?
I believe Each NF Ranger station has a volunteer application - you can help with a variety of tasks from admin, to clean-up!
Lots of volunteer positions are posted each year for almost every NF, BLM, and Wildlife areas. Some even come with free campsites
My closest NF also has a small non-affiliated club and we volunteer for trail maintenance about twice a year - it's a blast!
Get to know your NF volunteer coordinator.
Got to be lots of other good ideas out there....
"Climb the mountains and get their good tidings.
Nature's peace will flow into you as sunshine flows into trees.
The winds will blow their own freshness into you, and the storms their energy,
while cares will drop away from you like the leaves of Autumn."
— John Muir