Dgob,
I tend to get a gap sometimes at the edge between the platter and the Mat1. I correct this by applying the appropriate pressure to the Basis clamp so that when it is locked in place it doesn't dish up the record and the Mat1 with it.
I've also tried with good success placing a 1.9mm O-ring in between the platter and the Mat1. When the clamp is off the record, the Mat1 will float a bit. But when you apply force with the clamp, the Mat1 makes an even contact with the platter, while at the same time having a uniform contact between the Mat1 and the record.
Best,
iSanchez