We propose a simple microfluidic device for protein preconcentration based on the electrokinetic trapping principle. It comprises a narrow Nafion strip that is simply cut from a commercial membrane and is integrated into a molded poly(dimethylsiloxane) (PDMS) microfluidic structure using a guiding channel. Mechanically clamping the PDMS/Nafion assembly with a glass substrate results in a rapid prototypable, leak-tight, and easily disposable device. Our device preconcentrates negatively charged fluorescent proteins located at the anodic microfluidic compartment side of the Nafion strip within a few minutes and up to a concentration factor of 104. Moreover, we present a numerical study of the preconcentration effect by solving the coupled Poisson, Nernst-Planck, and Navier-Stokes equations for our type of device, which provides microscopic insight into the mechanism of preconcentration. The electrical field across the ion-permselective Nafion generates concentration polarization, i.e., ion depletion at the anodic side and ion enrichment at the cathodic side for both types of ions, with a local excess of mobile positive ions in the depleted concentration polarization zone, inducing a nonequilibrium electrical double layer in close proximity to the Nafion membrane. A voltage difference applied over the anodic compartment is used to generate the electrophoretic flow velocity of the negatively charged tracer biomolecules. This, in combination with the electroosmotic flow in the opposite direction, which originates from the fixed charges on the channel walls and the induced space charge near the membrane, provides the basis for the local preconcentration of the negative tracer biomolecules.