'chant' note entry

Hi all, I also sent this under 'contact', so if you are the moderator, I apologise for double-posting. I am trying to enter 'chant' notation where there are 12 8th-note values in the first and fourth bars, and 15 8th-notes in the second and third (some of these are 1/4 notes, but the total value for each bar is either 12 or 15). How does on go about entering a specific number of beats per bar that are not 'standard' time signatures? e.g. 15/8 (12/8 is there already) and 5/8 for the 'Amen' bar at the end, although I can just fill that in with rests.

I have temporarily used 9/8 followed by 6/8, but that is not quite what I had in mind.

Thanks in advance.

Derek