VEVOR Drawer Slides with Lock, 1 Pair 48 inch, Heavy-Duty Industrial Steel up to 500 lbs Capacity, 3-Fold Full Extension, Ball Bearing Lock-in & Lock-Out, Side Mount
Although I think they are designed for side mount I believe you absolutely could mount them flat under the drawer as I've actually done that on another project... that said it probably would affect max load slightly but honestly it would most likely have to be a pretty max load to take it to that limit... all in all these aren't the highest quality such as an accuride slide but they do the job just fine
I think mounting something flat could be challenging, but the bigger issue i see is weight, I do have some for a roll out truck bed, and they work well, but depending on the weight, I don't think it support too much flat.
what is the difference between
VV-SJTTDGDS-48IN ($118) &
https://www.vevor.com/drawer-slide-c_10537/heavy-duty-drawer-slides-500lb-full-extension-48-ball-bearing-cold-rolled-steel-p_010249134530 ($99.99)
Both SKU products have a locking function. SDSSSCTD48500OSGPV0 has two slide rails with a locking structure, and CTDG48YC500B00001V0 has one slide rail with a lock and the other without a lock.