Gather your supplies
1950s-style table legs with angled top plates (4)
18mm-thick plywood sheet, slightly larger than case size
Fabric to match case
You'll also need
Sheet of card; Helmar Super-Tac Glue; 16 screws (long enough to go through top plate, case and halfway through plywood); jigsaw; drill; clamps; metal ruler.
Place case onto card and trace around edge. Allowing for thickness of case walls, draw a second line, 5-10mm or more inside first outline.
Cut along second line to create a template that fits neatly in the base of case interior. Trim, if necessary.
Trace around template onto plywood. Clamp plywood to suitable work surface, then cut along outline using jigsaw.
Cut fabric, 40mm larger all around than the card template. Centre card on wrong side of fabric. Snipping into fabric allowance around curves (if any), and stretching fabric to ensure it is taut, bring fabric edge up and over to the other side of card and glue in place. Glue plywood onto interior base of case. Then glue fabric-covered card on top of plywood. Allow to dry.
Arrange closed case with the bottom facing upwards. To determine position of legs, using ruler, draw a centred rectangle on bottom of case. Position a top plate (with or without leg attached) in each corner of case and mark position of screw holes.
Open the case and, supporting it with anything stable that will fit inside, drill marked clearance holes through the case and partway into the plywood. Screw plates and legs in place.