1/24 scale Milk crate:
Hopefully you have a real one there. Measure it and divide by 24. (metric in MM is easiest) That is your dimensions. Build the outside frame out of 'L' styrene stock (angle iron) with flats across the top. Find suitable mesh for the checkered sides and bottom. Even fine mosquito screen would work. Fill these areas in with the screen from the inside to keep the corners nice and square / smooth. Prime with flat white first before the yellow or red.
1/24 scale cinder blocks:
Square styrene stock is notorious for being not so square. Get the dimensions and find 'C' or channel shape styrene stock with the same scale width. Build as a plastic cinder block, fill gaps etc. When happy with the dimensions / shape put a thin coat in CA glue on one side at a time and sprinkle talc powder (baby powder) on for texture. Once ready prime with automotive light gray primer and when its dry do a wash with acrylic Panzer gray.
Good luck with the scratch build details Bob!