===================================I'd recommend sheet metal screws instead of woodscrews for MDF,
because they are not tapered and won't tend to split the MDF.
On second thought, I'd recommend using baltic birch instead of MDF.
MDF gets eaten up to quickly at the front of the slot by erosion from
flying sawdust. that's my experience anyway.
On third thought, I just got a bunch of various sizes scrap lexan. I
think I'll see how it works as a zero clearance insert. :-)
SFWIW, scraps of 1/4" hardboard and double backed tape works for me.
Also, stainless steel coarse thread self tapping sheet metal screws
work for me.