I did a basic hack of my own from accidentally bending a cage… I think it only worked though because the sleeping mat I put in happened to be the right shape and is light weight. The majority of the work was being done by the Velcro strap holding it to the frame, the cage just kept it from slipping round into the cranks. Wouldn’t like to try it on the fork which would be my preferred position for a cage once I have a frame bag.
I have added a link to your blog from links page on my site btw. 🙂