Hi mate my roost is the std preston one and is attached to the outer leg of my rive tray, i have done mine one of two ways the first is to change the clip one rings on the tray and use d25 ones with 25mm round legs that are compatable with most other fittings the second is similar but use the d36 open system legs where you slot a piece of 25mm leg in the top and again you can use usual fittings. these can also be used on the back leg of a std box(not open) but you would have to knock off the open adjuster on the top of the leg(not easy but possible as I have done it) and re-fit once on the box.
Hope this makes sense and is of use