But, note as in the attached screenshot of a curtainwall with this panel: the panel will be distorted to match the size of the actual panel. (This is the same thing that happens with custom door panels.)
Notice the 3 x 6 panel flat on the ground thiwhich was saved as the custom panel compared to the appearance of the panels in the curtain wall.
Thus, it is essential that you model the custom panel with the exact dimensions of the desired finish panel if no distortion is acceptable. This may result in several custom panels, one for each required size.
Do pay heed to Dwight's advice: if there are many holes (such as a perforated metal), it is better to model as a solid panel with an alpha-mapped texture for rendering and a hole-pattern fill for line drawings as the polygon burden of thousands of holes could be extreme.
PS The screenshots are from AC 12. You did not say what version you were using. Always specify that in your question, better yet, in your signature (click the word "Profile" near the top right of this screen).
One of the forum moderators
AC 26 USA and earlier • macOS Ventura 13.4, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB