Monitor arms
Overview
Using a monitor arm with your pen display comes down to whether your pen display is VESA mountable.
If your pen display is VESA mountable, you can use a VESA compatible monitor arm.
If your pen display is not VESA mountable, there are some options
Arms for pen displays without VESA support
Smaller pen displays are about the size of a laptop, so you can try an arm designed to hold a laptop.
Here is an example: https://twitter.com/eyekoodraws/status/1596064399109726209
Here's some creative DIY solutions:
Wacom Flex arm
Wacom has designed an arm for their Cintiq line called the Wacom Flex arm that is specifically designed for their product. Not that this arm only works with specific Cintiq models.
Brad Colbow - Wacom Flex Arm Review Dec 10, 2018
Wacom VESA mount
If you already have a monitor arm you can use the Wacom VESA Mount for Cintiq 24 & 32 with it to mount those specific models.
Huion ST500 Desktop Arm
Huion has a monitor arm which appears to be a rebranded version of a generic monitor arm.
r/Huion - Huion ST500 Desktop Arm Review 2024/05/31
North Bayou
This brand comes up a lot when people mention what arms they use, but I don't have any personal experience with them.
North Bayou F100A
Ergotron
I use this brand a lot. My notes on Ergotron monitor arms
Ergotron - Install Ergotron LX Monitor Arm 2021/10/01
Ergotron - HX Monitor Arm Adjustments 2020/03/04
VIVO VESA monitor and touch screen desk stand
I've used this for a while with a 22" pen display and I really like it. My notes on this stand

XP-Pen ACS15 Ergo Stand
See this video
Wobble
All monitor arms have some amount of wobble. Some have more and some have less. But NONE of them feel "rock solid"
One thing that will help reduce the wobble is if the bottom of the pen display rests on something like your desk.
If you want to have ZERO wobble you need to use a stand with your pen display.
Resources
There's a write up on using tablets that includes a section on a North Bayou arm here: https://www.asianjoyco.com/resources-tutorials/digital-sculpting-tools
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