I use ABS sealed boxes for packaging, the ones I buy (Altronics H0310 & al) have mounting pillars moulded into the base inside. I've always found the pillars to be an inconvenience, they never suit the spacing of what I'm mounting and they take up a lot of space. The first thing I usually do is to remove them. It's an awkward operation, you can't get a file or sawblade in because of the sides of the box. I've done it in the past by drilling them down, that doesn't always work a treat because the drill tends to precess off the raised surface and you end up with an odd shaped crater wall that's just as troublesome.
Anyway, what I've found is that if you put the box on its side on a hard surface, lay the blade of a flat-blade screwdriver up against the base of the pillar and give the driver a sharp tap with a hammer, the pillar shears off neatly, level with the surface. All you're left with is a small rough round area, which doesn't impede surface mounting at all. It's worked every time with a couple of boxes, either it's designed that way or (more likely) the moulding operation creates a stress point on the base of the pillar that fractures. FWIW, hope it helps someone.