We have connections in our labs on LR that are short distances and the connection is fine. Its the rather expensive way to go 20 meters (as opposed to SR), but I believe it should work.
If you do need to attenuate, we do not have any recommendations It can get messy though. Suggestions is starting with a 10 DB pad and then check the power readings on both ends. Ideally SR is better for short connections.
thanks for your answer!