It was attached by gluing onto the bottom of the sideframe, and then soldering (Yes, Soldering...) the end pieces of the bottom strip about 1/2 way along the overhanging piece of the top strip. Geeze I'm crap at verbally explaining things. Its why I normally revert to the whiteboard before I open my mouth.
As built bogie on the left, modified bogie on the right. Its a short job which I think is worth the effort.