Measure the battery. Measure the panel you will use. Measure a gallon jug. Then add whatever shelves or such for other items you may want.

I found 6" wide was a real good number, its the size of a panel, the jug and fits my battery inside. Then i made one shelf the size of my starter, and put the battery in a compartment under it (the shelf is like 2" by 6" or so). I made a big shelf big enough for the radio and other stuff, and put a drawer under it. At the other end i left a big lip for the gallon jug.