Use one of your 8'' fins by itself. If not, I used to use a 6 1/2'' fin with lb side fins on my 7'10 which worked well. My 7'10'' gets used as a single fin now, love it!.
this is a template that works well for my MP3 designs , it works great with some small side bites, when side bites are in, I usually have a 6-6 1/2" single ( depending on surfers size, weight and waves to be ridden of course ), and when ridden as a single only, I push the fin further back in box, or up the fin size to 7- 7 1/2" fin......but this template gives good manouverability compared to some other singles I have used but it is a fine line between having it right ( placement in box and also right combo of size and with side bites ), and not quite right, quite a difference in performance.....but when it is right, it feels very sweet.....