Well sorry that you can not take the best possible answer. Since you wish to just have at me instead of taking some genuine advice fine. I can understand that. But if you want simple. USe GIMP or photoshop to create a flat model. Just use boxes and be done iwth it. Draw some lines for the wiring. Simpley put CAD is the best. Others do not compare.
Sorry you dont like the answer but it is the truth. Plus i moved this ot the Web Graphics are where design graphic software is discussed.