Neutral should not be switched. Ideally the disconnect box supports a neutral bar, but if not a suitable wire nut or any other code compliant wire connection method for in-box connections is suitable. 50A is not a small amount, though, so I'd shy away from wire nuts, as you should be using a 6 awg wire. Putting two of those into a wire nut requires a special wire nut. At this size you should instead consider a split bolt connector.
Another option would be polaris connectors, which don't require the effort needed to properly insulate a split bolt.
Wire nuts, split bolts, and polaris connectors are less convenient than a neutral bar. You don't have to worry about whether you've tightened and secured the wire nut enough, insulate the split bolt, or take up as much space as all three options. For split bolts, make sure you understand how to insulate it properly - it often requires several layers of two or more different coverings to do properly.
Also please be aware that ground and neutral should be separate at this point. If you place a neutral bar in there you might be tempted to make it look like the main breaker box, and connect both neutral and ground to the bar. This will interfere with the GFCI operation. If you use a a neutral bar make sure it is insulated from the box and ground. Some neutral bars are not insulated, so even if the ground isn't connected directly to the neutral bar, they might be connected due to the way the neutral bar is attached to the metal (grounded) disconnect box.