Have you any bits of timber and plywood lying about and the means to cut them to size? Can you make them from thin ply, say, 1/8th" (3mm) cut to size to fit in the length of the shelf?
I would suggest that the shelf does not fill the locker back to front. This will allow space at the front for sauce bottles which are best stowed upright in case they leak.
You would need to screw or fix 10 or 12mm deep stiffening strips along the front and back edges with glue such as "No Nails", otherwise they might sag when loaded. Take the stiffening strips right to the end of the shelf then arrange the size and position of the end supports to fit inside the stiffeners. That will position and anchor the shelf. The end supports could be made from 19mm material, I would suggest that, when fixing to the locker walls, they are screwed through from the adjacent locker. This will give the screws a hold in the support rather than in the locker wall. If you adjust the length and position of the end supports to fit between the stiffeners, the shelves can be lifted out if you don't need them or you want to stow something which is too tall but cannot be laid down. The stiffener strips and the end supports could be made from timber, chipboard or MDF. As they won't be seen there is no need to paint or varnish them.
If you don't want to screw the supports to then walls of the locker, make the shelf as suggested above but add four legs at the corners to make them free standing.
Hope that helps if you cannot find any shelves to purchase.