Yeah, I managed to get the Sigma TS-41 collar years ago, it's a much better fit on the 300/2.8 ( I think it should have been standard on it personally ) I then have lens plate attached to it to mount on the gimbal.
They are rare as hens teeth these days, but I actually see one instock at an Aussie store.
Buy Sigma TS-41 Tripod Collar and shop at CameraPro Australia.
You may be able to find a long lens replacement foot that has the drop down curve in it, but most of those use 4 screw mount, maybe only using two screws will be enough, but the added strain of the extra length would be a concern.
The only viable option may be as mentioned above, get some one to make a spacer block and use some very high tensile longer bolts to mount it.