I'd recommend using only two or three knots per character. They would be multi-turn overhand knots, 1-4 turns, representing row, column and letter (if needed).
Numbers would be represented by two knots:
1=11, 2=12, 3=13
4=21, 5=22, 6=23
7=31, 8=32, 9=33
*=41, 0=42, #=43
Letters would get take three knots:
a=121, b=122, c=123
d=131, e=132, f=133
g=211, h=212, i=213
...
p=311, q=312, r=313, s=314
etc.
Use the Zero button for "space" (421).
Or if the scout knows Morse code, use single and triple overhand knots to represent "dits" and "dahs".