budget macro lens

Remember you need a lens with aperture ring if using cheap 'dumb' extension tubes - an M50/2 or similar off-brand lens would probably be the cheapest good option.

Even more interesting (to me anyway ), if you bought two sets of those extension tubes, you could use lenses not just off brand but off mount too, they look like modular tubes with interchangeable ends. So any old cheap sharp manual lens from anywhere....

Just scratch off that paint so the contacts can errr .... contact.

You could go old school and stick an enlarger lens on there too? With the right adapters, PK to M39 adapter £7.99 90mm rodenstock £13, tubes £5.49.