Why bother? I've had my 250 SE-L for four years now and the space saver has never been used. If it has to be it's only to get you home or to the nearest tyre services.
It seems that Lexus/RAC disagree with you. I had a blow-out on the motorway and called the RAC (cover provided by Lexus). They hauled my car to the nearest service station and then refused to take me to my end destination as I had a space saver in my car. It was 01:00AM in the morning and I had a family travelling with me (including two children under 10) as we were going on vacation. Lexus/RAC told me that it was safe for me to complete a 230mile motorway and A road journey using the space saver to get to my end destination where I would need to find at my own accord a tyre replacement service.
I obviously refused to do this and the RAC/Lexus people couldn't see why this was an issue, as far as they were concerned it was safe for me to do this. I ended up opting to pay some third-party to come and fit me a new tyre which cost me £460, the replacement tyre was inferior and not even the same thread/make as the existing tyres which meant I had to change my other front tyre at my own cost aswell.
I am yet to write a letter to Lexus and RAC but I really don't see the point as there is absolutely nothing they can do as they not just cost me a stupid amount of money but also ruined the start of our family holiday.
I can't see why you think Lexus/RAC disagree with me, as I said "to get you home or to the nearest tyre services" Obviously the tyre service facility would need to be open to provide you with a repair or replacement tyre, just your bad luck there aren't that many open at 1 a.m.
I've got a full size alloy spare in the IS200, which I've owned for 8 years. In that time it's been used once.
The reason I said they disagree with you is because you said space savers will get you to your nearest tyre service or home which I totally agree with.
Lexus/RAC are saying you can use the space saver for a 230mile journey which both me and you know is nonsense regardless of the time of day.