I took the liberty of using Dmitry’s pattern and made a version without perforation.
Source Atle Mo
A bit of scratched up grayness. Always good.
Source Dmitry
The green fibers pattern will work very well in grayscale as well.
Source Matteo Di Capua
A mid-tone gray pattern with some cement looking texture.
Source Hendrik Lammers
A seamless pattern based on a square tile that can be retrieved in Inkscape by selecting the rectangle and using shift-alt-i.
Source Firkin
Background formed from the iconic plastic construction bricks that gave me endless hours of fun when I was a lad.
Source Firkin
ZeroCC tileable stone texture, edited from pixabay. CC0
Source Sojan Janso
A seamless pattern formed from a square tile. The tile can be retrieved by selecting the rectangle in Inkscape and using shift-alt-I.
Source Firkin
Zero CC tileable wood texture, made by me procedurally in Neo Texture Edit.
Source Sojan Janso
Tiny, tiny 3D cubes. Reminds me of the good old pattern from k10k.
Source Etienne Rallion
A seamless pattern the unit cell for which can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
Remixed from a JPG that was uploaded to Pixabay by theasad121
Source Firkin
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
Derived from an image that was uploaded to Pixabay by Kaz
Source Firkin
Square design drawn in Paint.net and vectorized in Vector Magic.
Source Firkin