Baby roll-up slats
15 slats made of solid, untreated wood, each approx. 5cm by 1.6cm and approx. 3.8cm apart, metal-free connected with robust tape that prevents squeaking and creaking because the slates are on top.
These 70x140cm of 20mm thick beech slats, at rational gaps, are surely more than your baby needs. But there is no harm in having bed slats that are this stable.
The smaller size 60x120cm can be ordered at the same price (please note so in the comment field at checkout).