Is the round clip mount still screwed onto the back of your SDS100?
Both the round clip part and the actual clip itself can be purchased from Uniden Parts. Sorry, I don't have the links but look for the parts under the 396T/XT models and they will fit the SDS100. The 436HP clip should fit as well. I'm not sure if the 436HP or SDS100 radio models are listed for parts purchase yet but the older models above will work for the clip and at least one should also have the round clip mount if that part is also missing from yours. I don't think Uniden sold the set with the round clip mount and the clip together so they must be purchased separately if both are missing.
Also, pay attention to the look of the actual belt clip. At least one of the older scanner models or the Marine radio had a beefier belt clip as could be seen in the parts picture. I'm not really sure they were stronger but they look like they could be! Both fit the same round clip mount.
Another thing, Uniden sold plastic and machined aluminum round clip mounts. If yours is missing, try and find the aluminum version as it will definitely be stronger. Some may come with machine screws though while the SDS100 uses more of a sheet metal type screw. The actual round clip hangar on the back of the SDS100 has always been the better aluminum type so hopefully yours is still on the radio.
At least one of Uniden's Marine portables also used the same clip.
Somebody will probably post some links to the parts here.