After several hours messing around today and falling onto a concrete floor I have finally settled on what I think is a great suspension system. (Time will tell).
I started with 1.8mm Marlow dyneema throwline. (The yellow stuff) and made a UCR. It worked flawlessly once you get over the learning curve.
The problems with the UCR is it's quite time consuming to make. (Field repair would be a nightmare).
You give up over 2.5 feet owing to the constrictor part and the locking mechanism you use to maintain tension. (In my case a prusik)
They can be finicky. But this gets better with frequent use.
If you decide to accompany your UCR with a dyneema softshackle in the same diameter these things require warm hands and great dexerity. (Not so good in cold wet weather)
The fibres do not hold up well to repeated opening and closing. Sharp fingernails can tear a few fibres here and there.