I have a 5.5" High-Back Banana "Little Chopper". What I do is glue a strip of flat-sided leather shoe lace to the base plate with contact cement. It's just wide enough that when I butt the feather up against it and cut the feather I get a nicely shaped 4 1/4" Nanner that is just about 5/8" high. I fletch with 4 of them and get great flight. They also look good, to me anyway. The leather lace can be pulled off if I need to cut the bigger ones again. Not sure if it works well with 3 fletch, you might have to use a full length low-profile for that.
3 Rivers just started selling a Low Profile Nanner Chopper. It measures 5 3/4" base, 3/4" tall. They get $20.50 for it.
I have a bunch of full length around. Left and right. If you want PM me your address and if you want right or left, and I'll send you enough to do-up a few arrows to see if you like them. That is if you use four fletch. If you like them then you could invest in a Chopper.
Another option is to trim full-sized Nanners to size with scissors.