I have had the occasional enquiry about how the standard WiFi is fitted to the t620. Here's how it is installed on my t620.
The photo below shows where the two WiFi antenna end points are fixed - at the top left and bottom right looking at the unit. The (black) wire to the top-left termination is run between the outer plastic case and the internal metal chassis following the route marked in red.
The next photo shows the top-left termination. You can just about make out the black wire (on black background!) coming up behind the front panel.
The final photo shows the bottom right termination. You can see the black and white antenna wires entering top left. The white wire goes to the termination that appears in the photo. The black wire heads off to the top corner and is routed along the bottom of the housing between the plastic case and the metal chassis.
The photos above show the routing for the standard HP part. The photo to the right shows a little more detail of one of the terminations. The dimensions of it are 25mm x 5mm x 9mm excluding the tab with the mounting screw hole.
If you use an antenna stripped out of a laptop then these details at least give you a starting point as to how you might try and fit it to the t620.