Generate a random, fixed length BigInteger in Java

I am attempting to generate a random 20 digit BigInteger in Java.

So far, I have been able to generate a fixed length hex value.

    // generate a user-specified number of random bytes
    public void getRandomNumber(int count) {

        byte[] bytes = new byte[count];
        new SecureRandom().nextBytes(bytes);
        String random = new String(Hex.encode(bytes));
        System.out.println(random);
    }

This is ideal for a fixed length hex generation. But I struggle to find an efficient method of doing this for a decimal representation; when converting from hex to BigInteger(hexValue, 16), the length will vary.

I have considered trying this by setting upper and lower bounds, or by generating something big enough and trimming to the desired length, but those methods do not feel very clean.