You'll probably need to shim it to get it flush with the top plate if you're using the low profile. With the cutout for the 510, a standard thickness (~2mm) top plate would be best.
And looking at mine from the inside, that hole for the 510 is huge, about twice the diameter of the Vartitube 510 I have here, and looks like its press-fitted (I could be wrong).
As a side note, I found a stainless washer at Home Depot that would work great, ON TOP of the Varitube (JB Weld?). It's just a hair smaller in diameter than the 510 cutout on the top of the K-Flask, and the 1/4" ID is just a hair larger than the 510 connector hole, and it's about 1mm thick so it should sit flush with the top plate, sitting on top of the Varitube connector. It would end up looking like that Waidea Flask, the silver one with no air slot. At $0.11 each, what the heck, I picked up a couple. Should polish up halfway decent too, being zinc. Picture shows the rounded side, other side is flat.
http://www.homedepot.com/p/Crown-Bolt-1-4-in-Zinc-Plated-Cut-Washer-807216/204633088?N=5yc1vZc2ck