How to add some random data at certain places in a file

With my Computer, it runs a Linux kernel and bash, i will make a technical-experiment.

I have a file, file.dat, size = 1.048.576 byte.

I will add at certain places in file.dat some irregular (1…8 byte) random data from /dev/random.

How to do that and how to remove the random data later?

By certain places i think at the Fibonacci Series, i found some scripts to generate this numbers:

#!/bin/bash
clear
echo "Program to Find Fibonacci Series"
  echo "How many number of terms to be generated ?"
  read n
  x=0
  y=1
  i=2
  echo "Fibonacci Series up to $n terms :"
  echo "$x"
  echo "$y"
  while [ $i -lt $n ]
  do
      i=`expr $i + 1 `
      z=`expr $x + $y `
      echo "$z"
      x=$y
      y=$z
  done

The other one:

# Read the value of n from user
echo -n "Enter the number of terms: "
read n

# Initialize variables
a=0
b=1

# Print the first two terms
echo -n "$a $b "

# Generate the Fibonacci series
for (( i=2; i<$n; i++ ))
do
    c=$((a + b))
    echo -n "$c "
    a=$b
    b=$c
done

echo

The Fibonacci Series says where to add sometimes 1 … upto 8 bytes to add with random data.

I know this blows up the 1.048.576 byte file.dat , but that is what i am looking for.

Any ideas how to do that?