There is no requirement in a Frise (or Frise-type) aileron for a slot at the aileron leading edge. The only essential feature is that the upward deflecting aileron leading edge projects downward into the lower wing surface airflow, countering the induced drag from the wing with the downward deflected aileron (and thus opposing adverse yaw). Frise described one geometry that achieved this simply.
The precise shape of the leading edge is important since it is easy to achieve negative aileron force (aileron snatching) particularly at high alpha (low speed), although this effect can also be used to give low aileron operating loads.
All Elliots sailplanes had Frise ailerons, and the chief engineer (one Jim Cramp) used to adjust a section of the aileron leading edge wooden fairing until the test pilot was satisfied with the handling. No doubt in the 21st Century this would be achieved by CFD before the first test flight.