putting extra shelf in hot press

A few pics of the inside of the hot press would be great for us to (A) understand exactly what you're trying to achieve and (B) in helping guide you through the process.

SoulWriter

That is called Slatted shelving

You could use 40 mm screws

1 screw at each end of the each 40 mm x 20 mm.

To prevent the end of the batten from splitting, you could drill a 3mm hole to take each screw. Also using a cordless screwdriver, set it to anti-clockwise and push the screw in to the 20 mm batten. This should prevent splitting.

Just try it

If SoleWriter is fixing battens to the side walls of the Hotpress to hold the slatted shelving, then 45 mm x 20 mm battens fixed to the studs with 60 mm screws will be perfect.