Tidewater

I am wanting to get rid of my runflat tires but am struggling with the idea of spending almost $1000 to retrofit the trunk to accept a spare. I thought someone had written once about a space saver tire made for another auto, but the rim size and lug size would fit the sc430 and the tire would fit into the existing well in the trunk without any retrofit. Is anyone familiar with this? Any suggestions would be appreciated. Thanks.