LetterBox.cs

using System;

namespace LetterBox
{
    class Program
    {
        static void Main(string[] args)
        {
            foreach (var arg in args)
            {
                var array = new char[arg.Length * 2 - 1];

                array[0] = arg[0];

                for (int i = 1; i < arg.Length; ++i)
                {
                    int ii = i * 2;
                    array[ii - 1] = ' ';
                    array[ii] = arg[i];
                }

                Console.WriteLine(new string(array));

                for (int i = 0; i < array.Length; ++i)
                    array[i] = ' ';
                
                int last = arg.Length - 1;
                for (int i = 1; i < last; ++i)
                {
                    array[0] = arg[i];
                    array[array.Length - 1] = arg[last - i];
                    Console.WriteLine(new string(array));
                }

                array[0] = arg[last];

                for (int i = 1; i < arg.Length; ++i)
                    array[i * 2] = arg[last - i];
                    
                Console.WriteLine(new string(array));
            }
        }
    }
}