Thanks JDFan> Thanks both of you. I've seen some bartop art that is exactly to size of each panel> Anyone know how they do that ?
Do you have a pic of the side of the cabinet and a ruler or 2 ? -- If you do then it is fairly easy to make a template.
1.) Measure the widest and tallest part of the side of the system
2.) open Photoshop and load a new document and set it for about 2 inches larger and wider than the measurements you got (ie. my current build is a bartop that is 22.5 inches by 15.5 inches on the side so I created a 24.5 by 17.5 inch 300PPI document as shown HERE -

3.) Add 4 new grid lines 1 inch inside each border using View - new grid line (this creates the box the size of our measurements -- (ie. for mine the lines are horizontal at 1 inch and 16.5 inches (1 inch in from our 17.5 inch document opened and 15.5 inches wide matching our measurement) and at 1 inch and 23.5 inches vertical for same reason) as shown HERE :

4.) Now import the picture of the side and use transform to adjust the size to fit in the frame we just made -- (Ctrl - A to select the imported pic - then Ctrl T ( to open Transform ) - grab sides to adjust the size of the image as needed until it fits the 2 measured lines on the placed guide lines as shown HERE -

5.) Use quick mask (or other selection tool you are comfortable with ) to outline the side panel for creating the template (zoom in as needed and clean up the selection until satisfied as shown here :

6.) turn mask into selection b toggling quick mask - hit Ctrl-Shift-I to invert the selection (so you now have everything except the side panel selected) and press delete (this leaves just your side panel template sized and ready for use as shown Here -
7.) GO to Select - Reselect and then Ctrl-Shift-I again to select the side panel and use the brush tool with a large brush and any color you want to fill in the side panel template to make selecting it easy ( I used Red) and you now have a side panel template to size ready for use in making your side art so save this as a template and reopen and start creating your side art.
Final Pic of ready made template of side panel made to size of the arcade -