How would I put a ceiling up here?! What would I use? Would plywood screwed to those battens be too heavy?
Welcome to the Bunnings Workshop community @RKM. It's fantastic to have you join us and many thanks for your question.
You might like to check with the manufacturer of the roof system as to what it is capable of suspending weight wise. They should be able to supply you with information as to whether cladding with any material is possible.
I don't believe Project Panel 1220 x 2440mm 3mm Premium BC Plywood would be too heavy for the battens. You might like to add Woodhouse Weatherproof 42 x 18mm 5.4m DAR PR4 H3 LOSP Treated And Primed Finger Jointed Pine Mouldin... battens to provide additional mounting points. This will ensure the 3mm plywood doesn't sag whilst also providing more rigidity to the steel battens.
If you had a builder inspect the roof they might be able to tell you if fixing plasterboard to it was an option. Obviously, you would need to check whether the roof is structurally suitable for that type of weight as it would be significantly heavier than plywood. I would advise checking with a builder, structural engineer or the manufacturer first before commencing work.
Please let me know if you need further assistance or had any questions.
Welcome to the Bunnings Workshop community. It's a pleasure to have you join us.
Let me tag @RKM to make sure they are aware of your question. Are you attempting a similar project in your shed? If you are, I suggest following @MitchellMc's suggestion and asking your shed manufacturer how much weight your frame can hold if you have a similar set-up.
Would it be possible for you to post a picture of your shed inside? Once we see what it looks like, our members can then assess if it's possible to install a ceiling inside.
If you have any other questions we can help with, please let us know.