Insurance that is suitable for adding me to during Uni hols?

[Mighty Flex]

My mother's Seat Arosa Sport insurance is in need of renewing by the 20th, and I am interested in being able to be added as a named driver when I am not at uni. I will be 21 on Monday (woop woop) and passed my test in September.
The cost of adding me for a year, or having my own car, is ridiculous, especially given I won't be able to use it most of the time, so being able to be added reasonably for a month or two at a time would be ideal.
Are there any companies to try specifically that will be useful and reasonable?
Thanks for any replies.
-Felix
ps. I'm not thanking anyone who replies with "ZOMG! FRONTING, YOU ARE KILLING THE KTTTENS!!!" as appears to be the stock reply to anyone under 25 asking about insurance regardless of the actual topic or question! people fronting pisses me off, as I suspect the possibility that a named young driver actually being the main driver actually puts up the premiums for people in my situation.

[Engineer1]

Hate to say but I doubt being added for a month then removed then re-added won't save you much as admin charges etc will mount up. You may need to get the insurance through a broker rather than the normal online sites as you want a guarantee that the admin costs won't swallow up any savings.

[W00DY]

I know Tesco allowed my sister upto 90 days added on per year on my Mum's car, she just got my mum to ring up and add her on for a week/weekend when necessary. Not sure how much it cost but it was definitely significantly cheaper than a full years cover.

[Mighty Flex]

thanks guys, 90 days over the year would probably cover me, are there any other insurers that do this?