how DIY are you? get plain old wood, cut it to fit, use cleats all the way around drilled into the studs, and some other supports on the ends and middle span portion. paint or stain.

or buy the ready made stuff. I'd prob do a combo of both.

might as well make it look nice. I'd also do something like this wood countertop

or find some remnant granite or laminate to use. maximize every inch

so, do some research and spend a little extra to get quality shelving and organizing. you'll thank yourselves years later.

Do those wall cleats like the first poster pictured on the wall. Than, purchase some 1x6 boards. You will have to cut them on the angles of the walls, like / \ on the ends...if you are looking at them on the floor.

Slide them into place horizontally...flat side, wide side up. Now, you can do that the total depth of your pantry closet, so that each board is a bit longer than the next, all the way towards your door frame, or within 6 inches of it...... Probably it would be a good idea to configure the space between each shelf to what you purchase: cereal boxes on their sides, stacks of 2 cans of vegetables, a PAM spray container, etc. Doing that you should be able to get as many as 5 shelves in your pantry, yet the top shelf, even with the top of your door frame, should only be the back 1/3 or 1/2 of your total area, so you can place items on it that are larger/seasonal...a nice sized roaster? or bulk paper towels for instance, between it and the ceiling... Use the floor as the bottom shelf, and if you purchase things such as gallons of pickles or heavier items, even large containers of cooking oil, or maybe you want a vegetable bin in there? It would make sense to place something on the floor such as a short nap bath mat, or rubber tub mat for those items that sometimes leave residue down the sides. You might start 2 feet or higher off the floor, depending on what you have to store, so you can use the floor depth. You might hold that bottom shelf one or 2 boards width off the door frame, as I suggest you must do the top shelf above your interior door frame..

This idea would give you alot of pantry space! and once you have the first set of boards cut, you likely have a pattern for the rest of them. I would use at least a 2x4inch board, screwed vertically on the right and left side, the depth of the closet, for each cleat side, and try to set the screws into the wall studs where possible on both sides.. This would insure that your horizontal boards could be off a bit short here and there, yet be safe on the cleats, as they just need to sit on them so you can remove if you have to adjust or just wipe them down at some point. Do put a nice vinyl shelf liner on each shelf to preserve the surface of the boards. The shelves can just sit on your cleats, likely, unless you intend to place a very very heavy item on one or 2, you could have them screwed into the cleats that are screwed to the walls on the left and right sides..