I have family in California. I’d like to build a rifle for my cousin.

Can it be Full Feature with a Sled single shot device somehow attached to meet the law?

I only have "featureless" configurations so am not familiar with all the different mag locking devices or how bolt release would work with the sled in place.

I don't know, and not a lawyer so typing out loud here. Since you are asking about "featured" rifles, here's the basic idea: "A semiautomatic, centerfire rifle that has a fixed magazine with the capacity to accept more than 10 rounds."  It's when they get into the definition of "fixed" that moved from "requiring a tool" to "disassembly."  Disassembly being the separation of parts of the action.  Here's the California legal definition of a semi-automatic firearm"
(hh) “Semiautomatic” means a firearm functionally able to fire a single cartridge, eject the empty case, and reload the chamber each time the trigger is pulled and released. Further, certain necessary mechanical parts that will allow a firearm to function in a semiautomatic nature must be present for a weapon to be deemed semiautomatic. A weapon clearly designed to be semiautomatic but lacking a firing pin, bolt carrier, gas tube, or some other crucial part of the firearm is not semiautomatic for purposes of Penal Code sections 30515, 30600, 30605(a), and 30900.
(1) A mechanically whole semiautomatic firearm merely lacking ammunition and a proper magazine is a semiautomatic firearm.
(2) A mechanically whole semiautomatic firearm disabled by a gun lock or other firearm safety device is a semiautomatic firearm. (All necessary parts are present, once the gun lock or firearm safety device is removed, and weapon can be loaded with a magazine and proper ammunition.)
(3) With regards to an AR-15 style firearm, if a complete upper receiver and a complete lower receiver are completely detached from one another, but still in the possession or under the custody or control of the same person, the firearm is not a semiautomatic firearm.
(4) A stripped AR-15 lower receiver, when sold at a California gun store, is not a semiautomatic firearm. (The action type, among other things, is undetermined.)

So with a sled, as described in the video, it locks back, stops the action and doesn't reload the chamber.  So it doesn't, to me, meet the definition of semi-automatic.  However, if one removes that magazine and/or the sled piece, you now meet the (1) definition.  And being featured, it's an illegal configuration.  If the upper and lower are completely separated, it's not an illegal configuration, the (2) definition.

I would suggest full featured with a mag lock and bolt release that functions with the sled in place.  That avoids any confusion on clearing, removing the magazine, etc.  It would be extremely easy for someone to release the sledded mag with an original mag release in place

I think of the quote from Hunt for Red October: "Sure, but why would you want to?"

If you're going to fix the magazine in place according to the standards of the law and regulations promulgated therefrom, you might as well use a ten-rounder rather than a sled or single-round magazine.  When fixed, it doesn't matter what features the rifle has.  Just make sure the rifle is at least 30 inches long in its shortest configuration, the barrel is at least 16 inches long, and the magazine does not exceed a ten-round capacity.

Thanks.

@DanishM1Garand

Also, fixed does not require "permanence".  In order to be fixed, it just cannot be removable without disassembly of the action, which means removal or separation of components needed for the weapon to function (like separating upper and lower, removing FCG or BCG parts, etc.).  There are a number of different mechanisms invented for this purpose, although I'm not up to date on them, since I'm just barely building my first AR and while in CA, it will be "featureless".