VEVOR Drawer Slides with Lock, 1 Pair 60 inch, Heavy-Duty Industrial Steel up to 500 lbs Capacity, 3-Fold Full Extension, Ball Bearing Lock-in & Lock-Out, Side Mount

The 500-pound slide rail is not removable for safety reasons and needs to be installed or disassembled as a whole.

Our slide rails are side mounted, just screw them on. No brackets are provided. We recommend you purchase the brackets yourself.

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

No good flat. They bind up

It is recommended to leave a gap of 20 mm on each side.

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.

I used mine in a traditional drawer for my pick up truck bed. Not sure if the capture ball bearings work or are durable in a flat installation.