tikz - 3D letter

Question

I just have to repost or renew an old question. I'm still searching for a good solution for a 3D representation of a letter with changeable content. It will be used for many final products an so the basic idea have to be perfect. It's not my first post about that subject, but I didn't find THE solution until now. This time I have a pattern and a working example for you (last time I didn't have this - sorry).

So I don't know how to improve this and bring it to the original pattern. Maybe one of you have the one idea to find a solution for this problem, finally. Thank you!

\documentclass[a4paper]{scrartcl}

\usepackage[ngerman]{babel}
\usepackage{amsmath}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage{tabu}
\usepackage{eurosym}
\usepackage{icomma}
\usepackage[absolute,overlay]{textpos}
\usepackage{pgf, tikz, pgfplots}
\usepackage{rotating}
\usepackage{graphicx}

\usetikzlibrary{
            arrows,
            shapes,
            shadows.blur,
            positioning,
            calc,
            intersections,
            decorations.text,
            backgrounds,
            plotmarks,
            shadings
}
\usepackage{xcolor,graphicx}
\usepackage[usestackEOL]{stackengine}
\newsavebox{\foobox}
\newcommand{\slantbox}[2][.5]{\mbox{%
        \sbox{\foobox}{#2}%
        \hskip\wd\foobox
        \pdfsave
        \pdfsetmatrix{1 0 #1 1}%
        \llap{\usebox{\foobox}}%
        \pdfrestore
}}
\def\mycell#1#2#3{\fcolorbox{#3}{#1}{#2 }}
\newcommand\makeply[3]{\mycell{#2}{\Longunderstack[l]{#1}}{#3}}
\newcommand\perspective[3][black!30]{%
\rotatebox{\myrotate}{\slantbox[\myslant]{%
  \makeply{#2}{#3}{#1}}}
}

\newcommand{\adresse}[3]{\begin{textblock*}{10cm}(6.8cm,6cm)
\begin{rotate}{-3}
\perspective[white]{\scriptsize #1\\\scriptsize #2\\\scriptsize #3}{white}
\end{rotate}
\end{textblock*}}
\newcommand{\betreff}[1]{\begin{textblock*}{10cm}(6.3cm,8cm)
\begin{rotate}{-3}
\perspective[white]{\textbf{ \scriptsize #1}}{white}
\end{rotate}
\end{textblock*}}
\newcommand{\inhalt}[1]{\begin{textblock*}{5cm}(6.3cm,9cm)
\begin{rotate}{-3}
\perspective[white]{\parbox[t]{9cm}{\scriptsize #1}}{white}
\end{rotate}
\end{textblock*}}

\begin{document}
\def\myrotate{1} \def\myslant{.1} 

\begin{tikzpicture}[rotate=-3,xslant=0.1,scale=0.9,every node/.style={xslant=0.1,transform shape,rotate=1}]
\fill [color=black!20!white,opacity=0.5] (0.15,-2.15) -- (13.15,-2.15) -- (15.15,-20.15) -- (0.15,-20.15) -- cycle;
\draw [color=black!30, fill=white] (0,-2) -- (13,-2) -- (15,-20) -- (7.3,-20) arc (270:220:9.9cm and 3.5cm) arc (-20:0:4cm and 15cm) -- cycle;
\adresse{Alpha Beta}{Gamma 123}{12345 Kappa}
\betreff{Bla Bla Bla BLA GGGGGGGGGGGGGGGGG}
\inhalt{BLA BLAB LABLABL \\
Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. \\
At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet.\\
Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum.}

\newpage

\begin{tikzpicture}[rotate=-3,xslant=0.1,scale=0.5,every node/.style={xslant=0.1,transform shape,rotate=1}]
\fill [color=black!20!white,opacity=0.5] (-7.85,5.85) -- (5.15,5.85) -- (7.15,-12.15) -- (-7.85,-12.15) -- cycle;
\draw [color=black!30, fill=white] (-8,6) -- (5,6) -- (7,-12) -- (-0.7,-12) arc (270:220:9.9cm and 3.5cm) arc (-20:0:4cm and 15cm) -- cycle;
\fill [color=black!20!white,opacity=0.5] (0.15,-2.15) -- (13.15,-2.15) -- (15.15,-20.15) -- (0.15,-20.15) -- cycle;
\draw [color=black!30, fill=white] (0,-2) -- (13,-2) -- (15,-20) -- (7.3,-20) arc (270:220:9.9cm and 3.5cm) arc (-20:0:4cm and 15cm) -- cycle;
\end{tikzpicture}
\end{document}