me I just use the lids as an extra layer of anti escape protection. then again I don't use the melamine wood. I get the regualr plywood( not the partivcal board stuff) and my saggign is cause hte actual shelves were made with the thin ( like 1/4 inch ) plywood. I not sure if I use the 1/2-3/8 thick boards if i have the sagging issue. I will find out in a couple years or so when I replace the racks. ( never expect more than 3-5 years out of wooden units.) I built mine as dyi but I set mine up were the tub sat on the shelf and the the next shelf was laid ontop and the braces were screwed to th side andthe shelve was then fasten to the top of the brace piece ( thus ths gave me a loose fit soI was not pushingon the flexwatt or damaging it when forcing tub in and out. ( hope to ge replace the shleve later on ( since wet they started to mold a little ( never stick washed/wet tile on a shelf they take forever to dry and allow the mold.)these shelves are getting a bleach scrub tomorrow since my niece does not have school and can help me by making sureI don't spill the bleahc water ( again) all over myself.