Do you really think the only shutter speed is that slow? More likely 1/60 sec. The old sunny 16 rule applies to ASA (ISO film speed). In full sun at f/16 shutter is 1/ ISO film speed.
If you have f/11 at 1/60 second that would mean the film speed needs to be ISO 30. If you use a 2X factor yellow filter the film speed would be ISO 60. FP4+ will work at 60. You might want to reduce the developing time by 10 %.
What is an Agfa Clack?