3D printer. That’s the thing you should be looking into first. That will give you more flexibility in anything to do with these kinds of toys. You can print a mount for the Handy that attaches to anything with a VESA adapter connection. You can print a sleeve holder that will hold a Fleshlight case, which itself can be adapted to hold almost anything within a typical size range. Or you can print something more custom for a particular sleeve.
When it comes to sleeves themselves, I’ve been heavily favoring Tomax ones for a while now. Especially the ones called “rich soft”. Beyond the sleeve itself, it’s possible to adjust its intensity with an elastic strap. For example, this, which you can cut to a particular length and add velcro to (using needle and thread, a skill you should acquire if you don’t already have it). Wrap that around the sleeve and it constrains the expansion, which affects sleeve tightness to a variable degree. Softer materials are less intense, but more realistic, and using the elastic allows you to increase intensity without going for a harder material that is, in turn, harder on you.