Head over to the ADVrider hacks section and ask there, there are a lot of Australians active who'll point you in the direction of a convertor.
The big item you'll need is a subframe. I havn't seen an F800 with a chair, so I'm guessing this will need to be made. Don't think velorex universal fitting kits are going to do what you need, they are designed for a Jawa sized bike and will turn, slip, break and generally be a PITA if you start to push them with something like the F. Once you have the subframe, the choice of chair will be up to you, there's usually plenty on e-bay.
I too run a Ural chair, they are tough, practical and better still parts are easy enough to come by. Platform chairs are a IMHO a PITA, for less weight saving than a couple of days of not supersizing your lunch, your kit gets wet, you have a security issue, it's about as streamlined as a brick and you can't rig a seat if you have a need to carry a passenger.
One thought; have you driven an outfit before? It's a completely different machine to a bike. If you havn't, get yourself to a Ural dealer or sidecar club for a test ride as item #1 on your to do list. If you hate that shopping trolley feeling, no point spending thousands setting up an outfit.