Check out misty fuse product to attach the cotton fabric letters. It is very thin and easy to work with. If you decide to use felt just use a glue for felt. No ironing needed. Is there someone else at church that could do lettering on a cutting/craft machine? Perhaps a local person could embroider the lettering for the banner.
Another option is to have the friend cut out lettering, etc and then you attach to the quilt and finish. Communicate with your friend exactly what you can help with on the project and your limited time.
Good luck on the banner.