This may look duplicate but I'm trying to draw a specific bump-decorated rectangle for an invitation card on the basis of the following picture:
So, I've been using the decoration library of tikz, but no matter the values I set for rounding corners, amplitude and segment, I can't match the corners of the decorated rectangle as I want. The result looks more of like a cloud rather than I expect it to be.

Here is the MWE:
\documentclass[10pt]{article}
\usepackage[utf8]{inputenc}
\usepackage{calligra}
\usepackage[paperwidth=15cm, paperheight=10cm, top=1cm, left=1cm, right=1cm, bottom=1cm]{geometry}
\usepackage{tikz}
%\usetikzlibrary{decorations}
\usetikzlibrary{decorations.pathmorphing, positioning}
\begin{document}
\thispagestyle{empty}
\begin{tikzpicture}[remember picture, overlay]
%Outside corners
\node[anchor=45,below=0.5cm, xshift=1.5cm] (SD) at (current page.45) {};
\node[anchor=135,below=0.5cm,xshift=-1.5cm] (SI)at (current page.135) {};
\node[anchor=225,above=0.5cm,xshift=-1.5cm] (II)at (current page.225) {};
\node[anchor=315,above=0.5cm,xshift=1.5cm] (ID)at (current page.315) {};
%Inside corners
\node[] (RSD) [below=10pt, left of=SD, xshift=20pt] {};
\node[] (RSI) [below right of= SI] {};
\node[] (RII) [above right of= II] {};
\node[] (RID) [above left of =ID] {};
\draw[thick, rounded corners=7mm, decorate, decoration={bumps, segment length=7mm, amplitude=5}] (SD) rectangle (II) [sharp corners] {};
%--(ID)--(II)--(SI)} {};
\draw[thick] (RSD) rectangle (RII) {};
\end{tikzpicture}
What am I doing wrong? Thanks in advance!
Ch.

