CAD designer: add 55mm water-bottle screw thread (GPI/SPI spec) to existing 3D model

PROJECT I have a 3D model of a fish-shaped bottle (a giant version of the Japanese soy-sauce fish bottle, scaled up for an art installation). I'll send the STL + STEP files. Your job is to add a precise screw thread to the neck so it mates with a real 5-gallon water-cooler bottle cap. WHAT YOU'RE DOING Add a SCREW THREAD to the neck of the existing model that matches the INDUSTRY-STANDARD 55mm 5-gallon water-bottle screw finish (the standard PET water-delivery bottle neck, GPI/SPI defined). A real off-the-shelf cap must screw on cleanly. I'll attach a photo and Amazon UK link to the exact reference bottle. REQUIRED SKILLSET This is a precise packaging/product-design job, not a generic 3D modelling job. Please only apply if you: (a) recognise the 55mm 5-gallon water-bottle screw finish as a defined industry standard, AND can source the GPI/SPI thread profile spec; OR (b) will source a real 5-gallon water bottle locally (reimbursed) and measure/scan it directly as your dimensional reference. Do NOT model from guesswork or photo-eyeballing. The geometry has to be precise enough that real off-the-shelf caps thread on cleanly. CONSTRAINTS - Threads sit in a short band at the TOP of the neck only — NOT down the whole neck. Match the standard bottle's geometry exactly. - Below the thread band, the neck must remain COMPLETELY SMOOTH for the rest of its length. - Do NOT model a cap — I have bought caps already. - Do NOT modify the fish body in any way — work ONLY on the neck thread area. - The model must remain hollow, watertight, with a single opening at the neck. - Final part will be SLA / clear resin printed — please design with that process in mind. DELIVERABLES - Updated STL (watertight, print-ready) - Editable STEP / source file I'll provide: the existing fish bottle STL + STEP, a photo of the reference bottle, and the Amazon UK purchase link. Fixed-price, one-off project. Please confirm your approach (which sourcing path you'll use) in your proposal.