Time - perhaps it would be an idea to set a warp to the area, and then mark out the area to be dug out (i.e. you just dig out the boundary, and then put signs in place to say "don't dig out further" (or something like that).
That might help with people that can only commit a little time to digging for you ... but as you know, if a fair number of people all commit a little time each, it'll be done in no time

On the mater of materials, you didn't say what you are planning on using - that might help people to start collecting for you