I don't think that most (or any) spacers are splined anyway, but uniglide splines are the same as hyperglide, it's just the lock-ring that's different. On uniglide the smallest ring <i>is<i> the lockring, which may look kind of silly, but will work fine. If you don't like it, you could just switch out the free-hub body to hyperglide from a trashed wheel.

Lastly, some freehub bodies were both uniglide and hyperglide compatible, check to see if yours is, and you might be able to use a regular lockring.