My truck had a 60" sleeper on it that mounted directly to the frame on 4 steel brackets and rubber bushings. I used the original mounts from the sleeper (4ea) and manufactured 2 more mounts of the same design out of 3/8" angle iron. The rubber bushings are the same the original sleeper were mounted on. I purchased those from the local Kenworth dealer. I will try to post some photo's of the mounts soon. My reasoning for mounting the box this way was to reduce road vibrations and frame flex to the box.Sorry for the delay in responding. I didn't see the question until today.
Can you provide me with any details on your rubber mounts? I am working on a similar conversion, and I am struggling with the box/frame interface. I like what I have seen from the various truck sleeper manufacturers but I am having trouble sourcing suppliers, and installation details. Any suggestions or info you care to share would be very much appreciated.