Upholstered Cube Ottoman

Build two identical frames from the legs and side frame rails — a rail at the top and bottom of each pair of legs, screwed and glued square.

Connect the two side frames with the four front/back connecting rails to form a cube. Check every face is square — this cube is the shape the slipcover gets sewn to fit, so a lean here becomes a wrinkle later.

Screw the three top slats across the top opening with even gaps between them, instead of a solid top — this lets a spill drain through instead of pooling under the cushion.

Cut thick cardboard (or 1/4" plywood, if you have offcuts) to cover the four side faces and staple it to the frame. This gives the batting and fabric a smooth surface to lay over instead of showing the gaps between the frame's rails and legs.

Wrap the whole cube in batting and staple it down snug on the inside/underside, smoothing out wrinkles at the corners. Batting softens every edge and corner before the fabric goes on, which is what makes the finished piece read as upholstered furniture rather than a covered box.

Sew a fabric slipcover sized to the finished cube, with a closure (zipper or a simple overlap) on the bottom so it can come off for washing. Set the foam cushion on top of the slats, under the slipcover's top panel, so the seat has real give.