Hi Christine,
The 406x is small enough to become fully boundary mounted by simply gaff taping it onto a hard surface without using the mount.
However, the available hard rubber disk boundary mount works nicely when you need some protection for the microphone (keeps it from getting crushed), and greatly reduces fabric noise if mounted under clothing, even more so than their clear plastic 'concealers' designed for under clothing use. It also provides a large enough surface area to tape or otherwise hold-down securely to prevent the mic from moving around. When inserting the mics in the rubber mounts, you do need to be careful to orient the capsule correctly so the ventilated side of the inner rectangular gold capsule under the outer grid faces out. There is a slight upper response change using the disk verses mounting the microphone directly against a boundary without the disk, but it's rather minor and many wouldn't notice it. They are available in black or white.
The BLM6000B is a dedicated boundary microphone with a weighted base under/around what appears to be the same disk, the weighted base has a threaded microdot connector in it, so the cable isn't integral to the microphone it is with the 406X. Should behave identically though.
I like the boundary mounts! Thanks for the reminder, I lost one and need to get another.
I posted a bunch of photos of the disks, including mic insertion into them at the link below-
http://taperssection.com/index.php?topic=70134.msg939783#msg939783Hope that helps.