I went to the Ye Olde Rocket Forum and found the side view of a Mini Katana so that gave me the shape of the fins
The Mini K had a fin span of 9.1 inches with a airframe of 2.6 inches diameter. So if I subtract 2.6 from 9.1 that would give me
the simi span of 2 fins divided by 2 that would give me simi span of 1 fin of 3.25 inches
I did a tracing of the fin shape and went to open rocket and inserted the fin shape on a 2.6 diameter airframe . then inserted 1 fin with a fin span of 3.25 inches
I then scaled up the open rocket from a 2.6 diameter to a 4 inch diameter.
Then I went to a pdf of a NCR catalog and read the 4 inch Katana total length was 67 inches and the 4 inch nosecone was 16 inches long attached to a 12 inch
payload compartment . So 16 + 12 =28 inches 67 inches - 28 inches is 39 inches or the length of the main airframe
I half to move the fins up a tad as the fins are not supposed to be even with the end of the airframe
Then I half to insert the motor mounts, centering rings payload bulkheads and recovery system