Cheers for the tips re branches, Paul; I'll keep a look out. I have asked a couple of people to keep an eye out, too.
You most certainly could use a good plywood. Or a plank of just about any hardwood, I guess.

The lovely little one that Pete gave me last year was a laminate one and it's a beaut.

I could definitely fancy making a few from plywood and solid ash, beech etc.

Sounds like you have the right tools for the job (I don't) to make it easy and quick.
If you ever get the chance, go on the Catapult Carnage YouTube channel. There you will find LOADS of tutorials on how to make catapults from various woods etc. There are also lots of good vids from other people.
The main ones that have helped me in terms of technique, making them, making and tieing bands etc. etc. are the Catapult Carnage, Catty Shack and Gamekeeper John YouTube channels. To tie the band set to the frame, I used the "wrap and tuck" method, illustrated by all these helpful people (and more besides).
Good news re that fulcrum. Do keep them safe. Is it up and running yet? Protek are nice, helpful, genuine peeps.
P.S......Ready made band sets, rolls of band (so many types and thicknesses available), pouches, tieing ribbon and other helpful accessories widely available from Catty Shack and Wasp Slingshots.
