properly sized plate holes should result in a firm hold on the switch. I imagine Cherry did a fair amount of testing on their switch design to ensure that the switch firmly holds onto a properly sized plate. Hot glue, as you mention, is a popular method for adding a bit of hold, and can be rather strong. Don't underestimate it.
As for other methods, you could shim the switch holes to make the fit tighter, say with plastic wrap or tape. Just don't break the switch trying to get it in a hole that is too small.
How solid your plate is depends on the material, and how many points of connection there are between the plate and the back of the keyboard. More connections from back to plate will result in a stiffer, more solid plate. How many connections you really need, or you can fit, will depend on layout. Using something like steel for your plate, even at 1.5mm thick, is plenty strong for a plate. The key thing to remember about plates is that they are only part of the main source of strength of a keyboard. They hold the switches in place, and add stiffness. The plate, however, relies on the casing/back of the keyboard to strengthen it.