Part number 7299.

A dot of caulk on both sides of the track, camouflaged with landscape material will do the same thing, without creating more noise through the tracks :)
Works like a dream for me !!
Tim

M3 screws at 20mm or 25mm will do fine for M track.

7299's are available on eBay; what I don't like about them is they are slotted instead of Phillips; their small size and short length (2x15mm) can be quite fiddly even with a correct size flathead/slotted screwdriver. As Harvey said, the 74990's (1.6x13mm) are close and even though the newer versions have Phillips heads their heads are small for M-track holes (they're meant for C-track). 7599's for K-track are even smaller (1.4x10mm). M3's with a flat countersink head and Phillips slot may work but the head diameter may be large enough that it sticks up. Since the 7299's are M2 they may be better because they can set lower without overly distorting the trackbed.