Seems there are only around 20 or so different space saver tyre sizes available. The road hero kits are the same limited number of tyres mixed across different hub sizes.
Given the age of the kits still out there for the 4200 and GS the tyres on the space saver kits are well beyond the tyres being in a safe usable condition imho.
I have no idea with the hump floor how much extra space can be accomodated. But to come up with a solution you need to start with the tyre sizes available and work back.
This is just an example but worth a bit more research.
To get the rolling diameter as close as possible you either have to go up a rim size with a lower profile tyre or down a size with a higher profile. You need to keep the rolling diameter within 2.5% so that it doesnt throw the speedo out, gear ratios and i would suspect possible traction control issues.
In this case i have just gone down a size as a quick example. In the picture below i have taken a space saver tyre size that is available.
The space saver tyre size used is 185x65x16 as you can see the rolling diameter is 0.8% difference.
The next issue would be having custom wheels made that are 16 inch with enough clearance to clear the hubs.
Your talking custom built if its possible but as a group buy 4 at a time for example i can see it costing around £250 for the wheel plus the cost of the tyre which is around £70 for the tyre.
If you went up a size to an 18 inch rim then the tyre widths available drop.
The nearest i can find is 145 x 70 x 18 which is made by bridgestone for £60 a tyre. Hub clearance would be much easier with an 18 inch rim. The picture below shows the overall difference in diameter would be inside the 2.5% allowed.
Im sure somebody with greater knowledge than me would be able to suggest possible custom wheel makers that could make a reasonable priced set of custom wheels for the purpose. If they are made from alloy then they can be cut from billet and be lighter than a steel alternative that just doesnt exist in the right size offset and stud configuration that i can find at the moment.