There ARE actually other options, depending on the extent you want to A) spend money B) do some work yourself
This guy modified his stock using a heat gun/plastic weld/some strategically placed screws, giving the otherwise unmodified rear end a much more UMPy look. You can also buy 15rd ProMag brand mags for the USC that are the same length as the actual 25rd double stack rounds, further giving the appearance of the converted ‘real deal’. I know I know, ProMag sucks but the general consensus on these 15s is that they are surprisingly reliable.
His has also been SBRd, which adds expense, but you could certainly skip that step.
Any combination of the above mods gets you significantly closer to the UMP look while keeping cash outlay to a minimum.