Yes everyone does this. Most people use MDF. I used particle board because I didn’t know better at the time. Either way it’s meant to be sacrificed and replaced from time to time.
When you screw it down make sure you counter sink the screws so you don’t accidentally hit them with your tools.
After you screw it down make sure you run a surfacing a pass to make it parallel to your machine. They have bits specifically for this for example.... Whiteside 6210 CNC Spoilboard Surfacing Router Bit, 1/4" Shank https://www.amazon.com/dp/B071748JQN/ref=cm_sw_r_cp_api_i_e2FmFbV4229ST
Agreed with everyone else.
Make a router sled, something like this. Then buy a router bit like this (make sure the shank size is correct for your router) and plane that way. It really is the best way if you dont have a planer.
too big a bit on a small shank introduces lots of stress leading to broken shanks and loose bits wrecking work. you don't want that.
instead, go with a 3/4" surfacing bit w/ 1/4" shank for $29.
the higher cost alternative (but great quality and value) is a bosch 1617 kit and a 1-1/2" surfacing bit w/ 1/2" shank.
I use this and it does great for me.
Whiteside 6210 CNC Spoilboard Surfacing Router Bit, 1/4" Shank https://www.amazon.com/dp/B071748JQN/ref=cm_sw_r_cp_apa_i_bHbcEb8WXK6AV