and what a good idea to ask around the users..... I have a little experience with cart covers - I have had two made for my big Chinhda cart (the 'team' managed to lose the first one - not our best day!!).
Front access is obvious, and JW's idea regarding using the flap as a head cover can work well, so that pretty much dictates a vertical zip on either side of the front panel.
Rear access is also jolly useful, so you may well consider two vertical zips on the rear corners, so this now, by default, wins you access to the side flaps by opening only two zips.
You'll have to either design two covers, one for top shelf users, and another for those that do not use the top shelf, or take a pragmatic view about making only one, that accommodates the top shelf users, but will also work for those that dont. Also, you'll need to take a view as to how much stuff the top shelf users pile onto said top shelf, so you need to accommodate an amount of bulk being on the top shelf, with the cover going over the top of that gear.
Be aware of stuff that sticks out at the sides, 4 or 5 20m cables hanging out the side on a hook take up a lot of space, and the cover may want to accommodate that.
Also, be aware of vertical sticky up things, like boom poles, aerial masts, etc....I had to send my big cart cover back to have a bunch of velcro flaps fitted, to accommodate all the sticky up things!!
I have found a wide horizontal flap on the back, near the handle, is a good thing for when moving, and also for access to patch panel or back of machines.You could consider a flap that integrates within the handle, this would be a good 'anchor point' for the whole cover.
Think about the hem line of the skirt, too high and gear gets wet in sideways rain, too low and it rubs on the wheels and gets dirty or trashed.
The cloth that I used I bought from eBay and is called 'Vinyl Cloth'. It comes in various thicknesses. Thicker is better. If the sewer is not experienced with this material then warn them that it can tend to 'tear' in places like the end of a slit or cut, so, for instance, the ends of the side slits on a flap, the point where the 'hinge' would meet the start of the cut for the sides, can want to tear. These points need some 'fortification' or extra strengthening.
Velcro is a good thing generally!!
I used a local company, out of Elstree Studios near London, who generally make green screens and drapes. They had the industrial sewing machines that make easy work of the cloth. They were 'up for it' and I paid them in wine. Karl did a brilliant job, and I would definitely go back there again.
I'll be really interested to see what you come up with Rob - feel free to share drawings before you commit - I am sure that you will get great ideas from here!!!
Kindest regards, and I hope it goes well,