Having given my SIB a real bashing over the past few years I have tried various seating arangements and have finally settled on what I consider the best option.
The attached picture shows my setup. I have a 'seat' in the middle which is actualy used to mount my GPS and compass - not for anyone to sit on. I sit on the floor in the back to operate the tiller and my passenger (if any) sits in the bow. We both sit on the floor (mine is one of those inflatable floors). The reasons for this are simple: 1) it's comfy 2) it's safer. It is safer becuase when you are bouncing about you are much more secure, also it keeps the weight where you want it - low down.
If we are just pottering about then I may sit on the tube.
To fit a plywood seat or 'instrument platform' you need some seat webbing attachments for the tubes. Available here: http://www.inflatable-boat-supplies.co.uk/
Look under 'Quicksilver Inflatables' then 'Inflatable Accessories' (a google search on this is NOT recommended)...
You cut the plywood to fit (see photograph) and having glued the webbing bit on to the tubes you need a plastic bracket thingy to slot into the webbing strap. This can be made by cutting some plastic plumbing pipe and then bending it to fit using a heat gun. When it cools it will stay in shape (unlike me!). You then screw this to the underside of the seat, deflate the tubes a bit and slot it in. Reinflate the tubes and away you go.
Having crossed the Pentland Firth in my SIB and also had many encounters with aggressive seas I can definiteley recommend sitting on the floor and holding on!
Keith (give your SIB some welly) Hart