A stackable smelter variant that tessellates over exactly 3 foundations, both vertically and horizontally, without clipping. Designed in Mk2 blueprint designer, but will fit just fine in the Mk1, so I can reupload if someone really wants it at that stage. Total footprint size is 24 x 13 x 9 meters, but tessellates within a 24 x 8 x 9 area due to interlinking with the adjacent blueprint, so the last 5 meters of width only requires factory space for the leftmost smelter in a line, and only 1m of this is at ground level. 9 meters is unfortunately the absolute minimum vertical clearance for smelters. Allows creation of 3D manifolds in whatever shape you desire: 5 wide and 4 tall, 2 wide and 10 tall, a single vertical or horizontal line, or even something irregular, as long as each column is uninterrupted.

After placement, power should be connected across a floor. There is no easy way to connect power between floors with such a design, so I recommend using connections from a factory wall near the end of the floor, or building a "power pole" of wall connectors to bring power upwards to each floor. Belts should be connected between splitters and merges in the classic manifold fashion on the lowest floor; on upper floors, instead connect each floor hole to the immediately adjacent splitter/merger to send products up or down the column.

Variants and modifications: This variant has inputs and outputs both on the left side, when looking at the input end. Can be mirrored if desired, but note that you cannot just turn the splitters/mergers around or it will no longer tessellate vertically; you must flip the whole design. Can be trivially modified to use a constructor instead (literally just switch it out and reconnect belts/power, no other changes needed), which also allows for the upper foundations to be lowered by 1m (with some slight clipping) if fitting in exactly 8m vertically is important to you (it can be nice when indoors). Uses Mk4 belts for accessibility purposes, but this isn't an issue until Mk3 miners provided you make the manifold at least 2 wide and 2 tall (as this immediately splits a 600 belt into 300 belts before it ever touches the conveyors in the design). One might also find it more aesthetically pleasing to bring out the support columns to the corners of the foundations. The belts will need to be extended to allow wrapping around the new placements if you do this, increasing the footprint to 28 meters long.

I had some trouble finding a design that tessellates both vertically and horizontally, and it was not trivial to engineer in such a small footprint, so I hope this is useful to people!