Quantity   Component name
1 × XH-M548-B TPA3116-based bluetooth amplifier module (£10)
1 × JBL CS-742 coaxial 4" car speakers (£20 for two)
1 × (optional) solar MPPT 3S Li-ion charger module CN3277-based board from eBay
1 × Visaton KT 100V 4" subwoofer (£20)
1 × Passive radiator 85x59mm (£3 for two)
1 × 11.1V (6 cell) Li-Ion laptop battery or 22V vacuum cleaner battery e.g. aftermarket batteries from Lenoge (£9)
1 × 3S protection circuit module e.g. HX-3S-01 from eBay (£3)
1 × Carry handle e.g. £2.50 from eBay
12 × Disc magnets 5mm dia x 1mm thick neodymium £3 for 25
1 × TL431A Precision programmable voltage reference (£1)
1 × Various offcuts of 9mm and 12mm plywood, screws, glue, etc.