I’ve never had bosses on any of my five RDS installations. Just a smooth cut. If the cut is correct front-to-back, and the screws are correct, the RDS has no where to go and will be secure.
One only needs bosses, I believe, when there is an oversized generic cut that needs additional stabilization or as a base for fitting plates. I don’t like plates as they add a layer of complexity and raise the sight higher than it has to be—a compromise I don’t want.
I agree that a direct milling that creates a well-fitting pocket supporting the optic body doesn’t need bosses. I also think that when one knows the specific optic they are going to use a direct mill is the superior option.
Unfortunately, though we argued for it, direct milling is not an option for us (my Department). That said, we’ve had a lot of experience with successes and failures and while a plate system is not “ideal”, it can certainly be made to function reliably when mounted correctly and adds the benefit of being able to switch optics, which I’ve done on numerous occasions as I’ve acquired more optic guns and optics.
Specifically to the question posed by the OP, there is another option I didn’t think of before my original post.
First, you could remove the rear two (or all four, but if they are there I’d keep ‘em) raised bosses in the Sheild’s optic pocket (or have a gunsmith do it).
Depending on the dimension of the raised bosses you may also have to trim the front two to work with a Holosun (such is the case with the Glock mini MOS guns). When I had possession of the gun, for about five minutes...I didn’t think to look to see if the slides threaded holes for the optic were through-cut or blind. This could create complexity of finding correct screws but once you figure out which ones you need McMaster-Carr usually has what’s needed to make it work - and that’s if the factory-supplied screws don’t work.
Of course, that’s the benefit of the C&H plate - You don’t have to worry about any of the above stuff because the plate will come with screws.
Otherwise you could always buy a non-optic Shield Plus and have it milled for an optic. That will get you a more precise fit for more durable mounting. While in theory it gives you a lower mount you can only go so low if you still want backup irons (The Holosun has an integrated rear sight), the cost for the milling will increase the overall cost of the project, and the modification to the pistol will probably not be worth it from a value standpoint if you choose to sell it in the future.