Mike, good idea using rope caulk. Seals well and never hardens.
CTR, using the tender shell as a baffle can work but only if there are zero air leaks.
Lionel's CV Hudson tender has a gasket to seal the shell to the frame. Not many others do.
Plus the volume may be too big to do much good. The ideal volume for a few 2" drivers is actually only slightly larger than a paint can top. This determined by testing and using Thiele Small parameters when available.
Pete