Optical concentration obtained by light confinement bears unique features that can increase the efficiency of a photochemical reactor. A suitable implementation of this method for a solar reactor is a series of parallel tubular receivers sealed in a slab-shape reflective cavity, in which light is trapped thanks to a self-adaptive optical filtering mechanism. To predict the concentration in such a generic configuration, we had previously established an analytical model based on idealistic assumptions, which are not valid in our real configuration. Here, we use analytical calculations and numerical ray-trace simulations to investigate how the finite size of the latter impacts the prediction of our model and extrapolate design guidelines for minimal departure from ideality. We apply these guidelines to design an optical concentrator maximizing flux density on tubular receivers and discuss the upper bound to the method, as well as the benefits from its unique features. Accounting for practical and technological limitations, this method can provide optical concentration in the order of ten suns in our generic configuration.