The barrel band from the early S410E rifles (to fit its 16mm OD shroud) works nicely when floating the MPR barrel - giving around 1mm clearance around its circumference:
The barrel does sit very slightly low in the band but this is barely noticeably (it's exacerbated in the shot above by the reflections).
If you can't get hold of an original one I think Rowan do 400 series barrel bands in various sizes.
My sort-of MPR definitely groups tighter with the barrel floated - 10 shots at 50m:
Hello forum, btw