I did this a while back also with a unit out of an IS300. I took the easy way and just separated the unit and used the access hole in the old garage door opener holder and mounted the unit sideways and it fits fine. I think I had to enlarge the hole to the side a bit.
Also best to connect it to a switched source vs. constant power as that way if someone breaks your window they dont instantly have access to your garage if you park outside.
Thank you for the picture and the tip. I am going to get on eBay and buy one. It is one of the few things that the LC did not have, and we really would like to have. I did not know it is that simple to add this.