Break the task down into smaller bits do the excel last, the tray is made up of simple objects so look at code that draws say plines from calculated points, then how to use repeat to make multiple circles and so on, look at how you would draw it manually.
Re the tray using a dynamic block would be a good start as you make wider a cable is auto added.
The vertical and supports including welds can be done simply using lisp and ask questions, they would use the Polar command to work out the next point.
Code:
;example box
(setq L (getdist "enter length"))
(setq W (getdist "enter width"))
(setq pt1 (getpoint "Pick lower left"))
(setq pt2 (polar pt1 0.0 l))
(setq pt3 (polar pt2 (/ pi2 2.0) w))
(setq pt4 (polar pt3 pi L))
(command "Pline" pt1 pt2 pt3 pt4 "c")