they look like strips of HDPE or UHMW, which is a plastic that is slick and by having them on the side and bottom, the drawer is held in fairly tight but can still slide easily. wood runners would work but have more friction and could also swell with humidity and jam up the tolerance making the drawer hard to pull out. probably about 3/16" thick to be thick enough to get a good countersink for the screws without going all the way thru the material. just guessing...What are those white strips inside your boxes. I know they help the drawers to slide but what are they and why do you have them on the side of the box too ?
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