You are more the welcome ot cut it, if it is large enough.
The pre stamped holes will make it easier than building one, and you can cut or have your local steel company shear it.
To cut it, make yourself a cardboard template and razor knife it to fit like you want. Then position it on the aluminum petition and decide where to mark it out
It is easiest to have someone shear the sheet for you, after you mark it with a sharpie. And you can cut or trim it with a sabre saw and spray a bit of WD-40 occasionally on the blade, to keep the blade from building up aluminum so bad.