Make a random string in batch

I am trying to make a random string in batch, and I have no idea what to do. I have called a random string of numbers with the %random% function, but I have no idea how to permanently replace certain characters in my variable:

@echo off
set subkey1=%random%%random%%random%%random%%random%%random%
set subkey2=%random%%random%%random%%random%%random%%random%
set subkey3=%random%%random%%random%%random%%random%%random%


ECHO %subkey1:0=a%
ECHO %subkey1:1=b%
ECHO %subkey1:2=c%
ECHO %subkey1:3=d%
ECHO %subkey1:4=e%
ECHO %subkey1:5=f%
ECHO %subkey1:6=g%
ECHO %subkey1:7=h%
ECHO %subkey1:8=i%
ECHO %subkey1:9=j%

ECHO %subkey2:0=k%
ECHO %subkey2:1=l%
ECHO %subkey2:2=m%
ECHO %subkey2:3=n%
ECHO %subkey2:4=o%
ECHO %subkey2:5=p%
ECHO %subkey2:6=q%
ECHO %subkey2:7=r%
ECHO %subkey2:8=s%
ECHO %subkey2:9=t%

ECHO %subkey3:0=u%
ECHO %subkey3:1=v%
ECHO %subkey3:2=w%
ECHO %subkey3:3=x%
ECHO %subkey3:4=y%
ECHO %subkey3:5=z%
REM after going through all the letters, we just pick random ones
ECHO %subkey3:6=a%
ECHO %subkey3:7=g%
ECHO %subkey3:8=k%
ECHO %subkey3:9=o%

set key=%subkey1%%subkey2%%subkey3%
echo String: %key%
pause

So as you can see, I am able to ECHO different keys, but I want to save the new one to the variable (EX in subkey 1, I wanna replace all the 0s by a's, then SAVE that so if I look at my variable afterwards, I can see the new one, and be able to call that in different functions.