That is not strictly necessary.
I run my LR3 with 1/8" hardboard and it is fine.
From the docs
“The Three Strut plates were designed to be up to 6.35mm (¼“) thick. Hardboard or any similarly rigid materials work best here.”
You can also usually source 1/4" MDF material at the box stores (https://www.homedepot.com/p/Outdoor-Essentials-MDF-Bender-Board-Common-1-4-in-x-3-3-4-in-x-97-in-Actual-0-25-in-x-3-375-in-x-96-in-487404/202297474), or from supply houses that deal with construction materials. 1/4" MDF is often used for concrete forms, siding, etc. and you may find some wide enough to meet your needs