Aside from the issue of potential invasiveness that should be avoided, there's another problem with their plant list -- the mature sizes listed are quite inaccurate, at least not without judicious pruning and upkeep (and even then, it's a large difference for some of those species between actual unpruned mature size and the chronic stress-inducing size reduction suggested by the listed heights). They are correct, though, in reminding gardeners to account for a plant's mature width when planting to accommodate the two-foot clearance around the sides and back. Many large-statured ornamental grasses, for instance, get from about three to six feet wide at maturity, especially if the grass blades arch over into a fountain-like shape, so that means the plant's center needs to be sited even further from the two-foot clearance zone to account for this eventual reach.

Iris are indeed more compact and upright-growing, but won't offer much of a visual screen of the box itself, if that is your goal. (If you just want some aesthetic distraction, though, they may work fine.) If the site is otherwise suitable for Iris -- well-drained, sunny -- then you can certainly try some. You should probably still use a mix of species overall, though, in case a future pest or disease outbreak happens to weaken some of the plants surrounding the box, even if temporary. (Diverse plantings are almost always more resilient due to their differing tolerances and susceptibilities.)

Other plant ideas, assuming a fairly sunny exposure (in summer) with good drainage and no deer browsing to worry about, include:

• Little Bluestem (Schizachyrium scoparium) -- a native grass known for its more upright habit and smaller mature size than most other (non-native) ornamental grasses; cultivars like 'Standing Ovation' tend to be even more reliably upright; Indiangrass (Sorghastrum nutans) might work for a taller-growing choice and is also native
• native Spirea (Spiraea alba or latifolia; Spiraea tomentosa) -- they tend to be more upright than the non-native Spirea shrubs commonly grown
• a modest-sized vine growing on a trellis that encircles that rear three-quarters of the box, with the trellis placed at least two feet away from it since vines tend to hug their support; Trumpet Honeysuckle (Lonicera sempervirens) and Virgin's-bower Clematis (Clematis virginiana) are two native species that stay smallish, though even here, only one or two would be needed to eventually fill that entire span of trellising in
• a dwarf, columnar evergreen cultivar like Juniperus communis varieties 'Gold Cone' or 'Compressa'
• a slower-growing, columnar evergreen cultivar like certain selections of Arborvitae (like Thuja occidentalis 'Degroots Spire'), though over the long term they might get taller than you prefer and trimming them shorter at that point isn't feasible
• Rattlesnake Master (Eryngium yuccifolium) - native and available at nurseries, though considered extinct in the wild in Maryland

Many native shrubs and perennials of suitable height for this type of planting grow pretty rounded -- about as wide as tall -- so might work only if you planted them further from the box, which we realize you probably prefer to avoid given your interest in slim-profile plants. Unfortunately, not many perennials and even fewer shrubs stay relatively short while also staying narrow.

Miri