I need help fixing a plate to a side case rack.

[3Putt]

I'm looking for a clean way to mount an XM puck to my Hepco Becker side case rack. The rack is flat top and bottom with round edges. I'd like to fix a plate that I could attach the puck to or maybe a small box to put the puck in.

I don't want to use a RAM or tech mount. I'd like to be able to walk into a hardware store and come out with everything I'd need. I have no idea what's available and am looking for some help.

Here's a pic of what I'm trying to accomplish.

Thanks.