There is really no need for clamping. The way the wing is designed, the center joint does little to hold the wing together.

What is important is:

1) Get a good amount of epoxy on the back sides of the spars where the wing joiner will come in contact with them. I push some epoxy into the pocket at each spar and smear some on the joiner itself before installing.

2) make sure the spars one one wing have good contact with the spars on the other - again, clamping is not necessary, but they should be touching each other.

3) BE SURE TO FIBERGLASS THE CENTER

Lots of people who assemble ARFs ask if they should fibergalss the center of the wing, and I say, "Don't bother" because the wing was designed to not need it.

But the US is designed in such a way that the fiberglass is an integral part of the wing and must not be omitted.