I've seen the term mounting posts used to refer to the plastic posts inside of project boxes for electronic circuits. As well as being quite clear, it's the only term I've seen used, other than the words posts or standoffs.
However, there are quite a few terms that could be used (and I thought I'd seen one or another of these used in various project box articles, but haven't found any references):
• column, “A solid upright structure designed usually to support a larger structure above it...”
• pillar, “A large post, often used as supporting architecture”
• piling, “A structural support comprised of a length of wood, steel, or other construction material”
• pier, “A rectangular pillar, or similar structure, that supports an arch, wall or roof”
• stanchion, “A vertical pole, post, or support”
• pylon, “A tower-like structure, usually one of a series” (and other senses, typically for items that taper to the top)
• turret, “...a little tower, frequently a merely ornamental structure at one of the corners...”; another sense, “a tower-like solder post on a turret board (a circuit board with posts instead of holes)” may be better known in this context.