Hi Franky,
at present custom made fields can only be used in the “multi booking form entry template” and not in the header/footer of multi-booking forms.
This makes it difficult/impossible to handle multiple events with various custom made fields (i.e. holiday-weeks for children, where details referring to the parents, the child etc. have to be recorded, see also https://wordpress.org/support/topic/multiple-events-filling-out-the-booking-form-just-once?replies=6), as custom fields can only be added to the “multibooking form entry template” and would therefore have to be inserted in repeatedly (in the above mentioned example once for every week which is booked).
If it would be possible to add custom fields to the header template (where every field is inserted just once) the problem would be solved.
Very useful would also be the possibility to determine a different order of the three templates of the multi booking form, for example stating the form entry template first (referring to the above mentioned example the booking person would first select the desired weeks).