Surely the grounding thing shouldn't cause too much problem as acrylic cases use stand offs but a metal standoff to a plastic case isn't grounding anything. Is it an option to drill through the based and screw to the base itselfs?? I'm thinking, 1-2mm neoprane cut to the motherboard size, then drill holes through the mounting plate you have, followed by tapping the plate to take the screw (or a nut on the back). Then setup as plate --> neoprane --> mobo
For inexpensive makeshift standoffs, I just used small diameter plastic pipe, cut down to the needed lengths. Then I would just use some screws to fasten it to whatever backplate I was using. It would ground out if the board if the backplate was conductive and the sections of tubing raised the board enough to keep it from shorting out. Basically what jabin suggested.
Another idea: you may use small bolts (1/8'' or 3mm diameter) put them through your aluminum plate, put some nuts or washers in each bolt as spacers and use a nut to fasten motherboard. I had to use this way of mounting for a Gigabyte motherboard as the height of i/o bracket does not match using regular standoffs.
what about double sided tape? no really! if its just a micro-ITX board and it will be sitting horizontally, some foam double sided tape would work mint. a few long strips along the backside of the board would keep the board from shorting against the metal. and because its foam, it will mold itself to the contours of the back side of the board, maximizing contact with the board for the best possible grip. additionally, it adds vibration dampening. the only down side i see is if you have to remove the board, that sort of tape usually rips into bits when peeled off...so youd have to clean up both the MOBO and metal before you stuck the MOBO back down.
