FluidSurveys: Affix the auxiliary buttons to the top of the screen's viewport. This will allow you to have the "Save and continue later" button always visible for your long survey.

fluidsurveys-save-for-later-top.css

/* Save and Continue Later visible at the top */

.survey-buttons .auxiliary {
  background-color: rgba(51, 51, 51, .95);
  padding: 1em;
  color: white;
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
}

.fs-chrome .survey .survey-buttons .button-group.auxiliary {
  margin-top: 0
}

.fs-chrome .survey {
  padding-top: 100px;
}

.fs-chrome .survey-buttons .auxiliary input[type="button"].small
, .fs-chrome .survey-buttons .auxiliary input[type="submit"].small
, .fs-chrome .survey-buttons .auxiliary input[type="reset"].small
, .fs-chrome .survey-buttons .auxiliary button.small
, .fs-chrome .survey-buttons .auxiliary .button.small {
  color: white;
  margin: 7px 0;
}

.fs-chrome .survey-buttons .auxiliary input[type="button"].small:hover
, .fs-chrome .survey-buttons .auxiliary input[type="submit"].small:hover
, .fs-chrome .survey-buttons .auxiliary input[type="reset"].small:hover
, .fs-chrome .survey-buttons .auxiliary button.small:hover
, .fs-chrome .survey-buttons .auxiliary .button.small:hover {
  border: 1px solid white;
  background-color: #222;
}

.fs-chrome .survey-logo {
  display: none;
}