I've already been down this road...
1. Standard AR receivers are illegal, because they have features that meet the criteria set by various California laws.
2. Various (but not all) standard AR receivers, regardless of features, are illegal because they are named in Roberti-Roos or are listed in the Kasler v. Lockyer assault weapons list.
3. This means that, technically, the only AR receiver allowed in California would be one that does not fall into #1 *OR* #2. To wit, it does not meet the criteria, AND it is not one of the listed "assault weapons".
Now, pursant to your question:
Even if you could build one meeting #3 (by using an unlisted lower), it's still iffy because there's nothing stopping them from adding your unlisted receiver to the list at any time. That means the second a Mega or Stinger or any other unlisted lower shows up on the list, you're a felon, even if it doesn't have any evil features. This applies even if you modified a Mega lower to EXACTLY match a FAB-10 lower, it can still become illegal at some future date when some politician decides to add it to the "evil list" by name.
You can use a receiver that is specifically exempted by the CA DOJ, such as the FAB-10. Or you can use a receiver that, by its very design, fails to meet the criteria for an 'assault weapon', such as the DPMS single-shot receiver for example.
It's not possible to build one that has a 30rd magazine that requires tools or disassembly to remove, because then that makes it a fixed magazine, which is limited to 10 rounds pursant to section 12276.1, #2.
In other words, if it's detachable (defined as not requiring tools or disassembly) it's an assault weapon. If it's not detachable (defined as requiring tools or disassembly) it cannot exceed 10 rounds capacity.
Theoretically I think you could build an 80% lower so that it did not meet the evil features criteria. And since it's an 80%, it obviously wouldn't be on the list. However, I dont' like the idea of becoming a test case depending on the opinion of the cop who sees me shooting my "assault weapon looking thingie" at the range...
Here's a couple of threads (one started by me):
[url]http://www.ar15.com/forums/topic.html?b=3&f=4&t=171277&w=searchPop[/url]
[url]http://www.ar15.com/forums/topic.html?b=3&f=12&t=169340&w=searchPop[/url]
Redbone's right, you'd have a hard time finding an FFL willing to sell you a lower, unless it was a FAB-10 or DPMS single shot. You could however build it outside the state and then bring it in after it's been neutered. However, you'd be subject to the whims of those who add names to the Evil List.