I have, my pictures are fairly rubbish but ill try share how i did so.
I removed the optical and 2.5" tray completely.
Attached the 3.5" tray in the same location as you would with 3.5" HDD in place. I then used the supporting arm that is initially used for extra PSU support to support the weight of the 3.5" tray and keep it flat.
The last two steps are bit out there but do the job very well. I had rubber gromits from an old case ~3cm which screwed into the 2nd HDDs underside screwholes sat this on top of the cage then secured front and back sides with a large elastic bank to stop any noticeable movement.
Not perfect and if i was a bit handier metalworker i couldve done something more robust. But the space is definetely there, and im even confident it could handle a fair bit of transportation as it is.