One simple way to have a semi-transparent object that doesn't show its inside on Eevee Next, as is it was a half-opaque 2d layer on a photo editing software, is to use the Principled BSDF's Alpha input to control the material's opacity, combined with setting the material's Render method to Blended and disable Transparency Overlap:

If the goal is to have a semi-transparent object that doesn't show its inside, as is it was a half-opaque 2d layer on a photo editing software:

One simple way to do this on Eevee Next is to use the Principled BSDF's Alpha input to control the material's opacity, combined with setting the material's Render method to Blended and disable Transparency Overlap:

This method is AFAIK the fastest to render, but tends to look kinda shadeless.

If instead you want to only hide the backfaces but still be able to see the frontfaces behind the foreground faces, then keep the default Dithered Render Method, and instead use the backfacing node, invert it, and substract to it a value corresponding to the amount of transparency you want. If you want to input a value that corresponds to the Alpha or opacity behavior, simply use another substract node where the base value is 1 and the second value is your proxy alpha value: