(Up to user-created-add-ons)
The Y axis uses a metal-on-metal screw-in joint which is an extremely strong joint in the important Y-Z direction (i.e., keeping the Y axis perpendicular to the Z axis) but without bracing is quite weak in the X-Y rotational direction (i.e., keeping the Y axis perpendicular to the X axis). This means that any small knock can put the Y axis out of alignment.
This page provides some printable braces that can be used to keep the Y axis locked in position.
The smaller X-axis frame bottom pieces are not as important - they just stop the Y-axis moving to the left or right. However you can insert the frame bottom pieces without pulling apart your whole frame just by dropping the M5 slim nuts into the Misumi channel and then screwing the piece into the nut from above.
Also don't forget to put M5 washers underneath the heads of the two M5x8mm Y-axis screws (inside the Y axis channel) to ensure they don't bottom out in the frame bottom channel. If the screws do bottom out you won't get a proper metal-on-metal join even when you fully tighten the screw.