If you just need round holes on the frontpanel its easy to use a subpanel. If you need toggle switches its another story, you need to mill the slots for the switches on the fronpanel.
I have some cases like that and its really nice, especially for toggle switches (where you see the ugly nuts on a normally panel)
If you just have potentiometers or rotary switches it depends to the knobs you use. If the knobs do hide the nuts nicely imho its not worth to build it via a subpanel.
For the subpanel you can use a flat piece of aluminium, with standoffs, mounted together with the Frontpanel to case screws. Depends on your case. Can look for a pic.
Or like in the first post you use a L bracket and screw it to the bottom.