Lines of Effort
Effort California is Efforts Thumbhole and Bloomberg modified to meet California law regarding thumbhole stocks and fixed magazines.
The construction of a non-thumbhole stock is quite easy; follow the plans for Effort Thumbhole and add a few bricks where the hole is. You'll probably have to glue the stock together, lest you be accused of having an easily removed thumbhole. Try to let the glue dry before trying out your new stock - California law is not yet ready to deal with citizens with permanently attached evil multi-colored rifles made of plastic bricks.
The magazine question is much more difficult. Our two existing mag well designs, in Efforts 22 and Bloomberg are both plastic at this time. Both are left handed and do not operate like a normal AR mag latch.
Jack Squat's 80% receiver flats may help.
Take a careful look @ parts 5 & 13. Look familiar? Might they not be glued on the outside of a brick magwell, and perfectly line up the mag release & bullet button? As well as adding additional reinforcement to the upper's forward lug pin.
More expensive but less work would be this magwell for the VZ58 from Czechpoint. As you can see, it has a standard AR magazine release button that could be adapted to a bullet button release. One disadvantage is it only accepts aluminum magazines, and some plastic ones with some modification. Another disadvantage is that it's currently unavailable.
The more intriguing possibility would be a left handed hopper, a la the Japanese Type 11 light machinegun, of a 10 round capacity. Gravity feed might not work for bump firing, but should be good for semi-auto. It could be made of mostly clear bricks so the rounds remaining could clearly be seen, and it could be covered to keep dust out.
Thanks to Wikipedia Commons for the above photo.
Loki has pointed out that a single column hopper could be set up to take stripper clips.
One expensive possibility is this: Canis' AK M4 conversion. It would certainly reinforce the lower receiver and solve the bullet button problem.
We will be seeking out advice from other Californians as to how to meet the requirements of California law. Many thanks to the Musician for raising this question.