I have experience as a fiscal sponsor, though not for a 501c7. The key question is how to assure that the funds will be used for eligible charitable purposes under the 501c3. Otherwise, the form of entity is less important than the clear language and controls in your fiscal sponsorship agreement. Make sure that the 501c3 sponsor has discretion over the funds. Because the sponsee is already a legal entity, a Model C fiscal sponsorship probably works best. (and caveat, I’m not a lawyer …)