Sized automatically to match the tray's own current footprint, and shown beside it in the preview -- there's nothing else to size here. A screw-on lid's thread is likewise sized and adjusted automatically off the tray's own Wall thickness and current size (Diameter for Circle, Width/Depth for every other shape), so it screws on smoothly with no separate thread controls to fiddle with beyond Fit above. For every Base type other than Circle, the threaded neck and its own base plate print together as one separate piece alongside the tray and lid -- no glue needed: it's sized as a press/friction fit, so it just pushes snugly into the matching inward shelf just inside the tray's own rim (one Wall thickness down, own width set by Shelf width above) and stays put once everything's printed. The plain flat lid now gets its own inward shelf too, fused into the tray one Wall thickness below the rim (same Shelf width above) -- the flat lid panel is sized to drop right into it and rest there, rather than just sitting loose across the top of the rim, on every Base type including Circle. A tray too small or thick-walled for a real shelf falls back to the original full-size panel automatically.

Square/Rectangular bases only -- other shapes have no straight parallel walls for the sleeve to slide against, so this has no effect on them.

Stackable base (align multiple sleeves)

Adds a raised square boss to the underside of this sleeve and a matching recessed square socket to its top, so a second sleeve printed separately drops on and registers in place -- the way to go taller than one print allows. Turning this on resets the tray's own Base bottom (in Basics, above) to Flat, since the two features build into the same physical base surface.

A stationary open-front shell, sized automatically to the tray's own current footprint plus this Fit clearance, printed as a separate piece beside the tray and lid -- the tray slides straight in and out of it like a drawer, resting on the sleeve's own floor. The tray's own walls (including the front) are left exactly as you've configured them elsewhere in this panel; only the Handle above adds anything to the tray itself, mounted straight onto its own front face.