I created this 2 part mount to stiff and reliable mounting accessories on Steam Deck.
Flexible "bracket" is a mid-piece between Steam Deck and very stiff "clamp". Check also Valve Index controller mount as example of an accessory.

Usage:
Mount "bracket" on Steam Deck, notice small pins that should hide in the top screw holes (in the Steam Deck backplate). Slide "clamp" on the bracket until click.
Slide your custom accessory in the slot on the back side of the clamp. Lock it with "lockpin". There is space for 3 lockpins, so you can mount 3 different things in the clamp clot.

I added STEP and FreeCAD files for piece that can be used as base for your custom accessory.

I used FreeCAD-Link:
Version: 2023.114.26244 +5346 (Git)
Branch: LinkDaily
Hash: 734bfcf15661aea45b019c0588e70e33c33ee73c

You my want adjust clearance of the connection, this can be done with changing Tolerance of the "Pad" feature in the piece-for-customisation.FCStd

Creality

Ender 3 Pro

Yes

20-40%

Print clamp and bracket on the side for increased strength and flexibility.