How to define a symbol using its unicode number?

Question

I want to define a pair of delimiters that look like (( and )). According to this question, I am looking for U+2E28 and U+2E29. However, I have no idea how to use these to define the symbols I want. What I have tried:

\usepackage{stickstootext}
\usepackage[stix2,vvarbb]{newtxmath}
\DeclareMathSymbol{\lParen}{\mathopen}{boldsymbols}{"2E28}
\DeclareMathSymbol{\rParen}{\mathclose}{boldsymbols}{"2E29}

But what shows are strange symbols rather than I expect.

P.S. You can try $\Big\lParen bla \Big\rParen$ to see why I do not want to simply define

\newcommand\lParen{\mathopen{(\!(}}
\newcommand\lParen{\mathclose{)\!)}}

Answer 1

To appear, and in particular to grow with \Big, the font needs to define the character, including a recipe for providing size variants. U+2E28 is in the Supplemental Punctuation block of unusual text punctuation, it is not in any of the opentype math fonts that I tried

You might be able to use U+2985 U+2986

\documentclass{article}
\usepackage{unicode-math}
\setmathfont{Stix Two Math}
%\setmathfont{Cambria Math}
\begin{document}
$⦅x⦆ \Biggl⦅x\Biggr⦆$
\end{document}