Each of the ends are secured with 2 small plastic locking tabs that clip into the metal psu case inside the plastic shell. The only way to get the end caps off is by forcefully prying the edges without damaging the actual cap. One or both of the inner locking tabs will likely snap off. Be prepared for sore fingertips in the process. The tiny locking tabs aren't required afterwards anyway in a build like this, because the metal endcaps of the metal shell are taken off and chucked out, leaving no place for the plastic tabs to clip into anymore. The end caps will fit snugly into the plastic shell anyway, tight enough to hold. If a little extra 'snugness' is required, simply taping a thin layer of clear tape around the end cap insert will give it enough extra diameter to be very tightly inserted.
Cheers!