I guess it would easier if you would also make the tray along with the case. The tray is also metal and is conductive, the only thing that keeps it from shorting out are the placeholders you have between your mobo and the tray.
Cases come with plenty of screws and placeholders, so you could probably cannibalize some of it from your original case (if you kept the screws and all) and use your current tray as a template to position the placeholders. Then maybe you can just screw them on and walah.. new tray.
My 2 cents worth though.