I recently built an IKEA “Bestå” unit as an entertainment centre in the living room. A source of frustration was that IKEA UK do not make a slide-out shelf for their Bestå systems, and I wanted to put a record deck on one, so I had to improvise.
The standard shelf that fits into the unit is 56cm wide and 36cm deep. I needed something a little smaller. I got lucky, I had a table that was going to be scrapped, about 8 years old, and when I dismantled it, I found the width of the table top was about 2mm short of 56cm! I’d planned on maybe 55cm, but figured that if I could get it into the unit, and it could move easily, it would do. I cut it to about 34cm deep. If you are attempting this on a different cabinet, you would need a similar or greater depth than the Bestå in order to accommodate the drawer runner.