You will need some cardboard hemis, which can be obtained from some of the suppliers on here.
I made a dummy 3" shell a few years ago for that same purpose. I made it as if it were real but didn't go too mad on the pasting.
A pair of hemis for the shell & a McDonalds sauce cup for the lift.
I used sand which was actually too heavy, sugar would be better or maybe sawdust.
For the leader, I used actual QM but removed the black match & replaced it with 5 strands of painted string. I even pasted a string loop to support the leader. If you work on pro shows you will be able to get QM easily enough.
I labelled it correctly as cat4 but used 'dummy shell' as the effect description.
We used one of them on a UKPS stand at one of the MLE champs. My 12 year old was demonstrating how we safely load them into mortars & insert a (dummy) ignitor !
A good place to start, before you start with metals & colour comps is black powder.
Until you master your BP you will not be able to propel any of your effects into the air, unless you are happy just making ground effects.
I would recommend starting with perfecting BP & then from here you can make blackmatch, which is used in quickmatch (essential for shells & mortar based devices).
Once you have working BP, you could then make simple charcoal stars (Tiger Tail is easy enough) & when you have them working you can then move on to making a firework.
A good first one to do is a star mine, using the Tiger Tail stars. If you make a successful 50mm Tiger Tail mine, I guarantee you will be impressed & have a feeling of satisfaction.
Mines are very easy to make & are also very effective. They are a lot easier than shells & require less time.
My 50mm mines contain 60g of stars (approx 10mm in size) with 15g of BP in the bottom for lift. They go with a fair old bang & propel a column of stars a good height. This size is not something to do in a small garden however, you will want to be at least 40m away from other people & property.
You can always scale this down to the size of your site. I also make 30mm mines, which are more suitable for a (larger) garden. They contain 10g of stars (5mm) with 3g of lift.
The key is to get the basics first & then work your way up.
Looks a nice venue Phil, should we not post links to other local pages/sites to announce our event in the area? or contact the town cryer or or perhaps just announce the firework display details for the local public to attend? = good opportunity for some publicity!
Also, it might prevent the locals from complaining like last year !!
If I can get down I will see what I can add to the show possibly if it's wanted.
Why don't you bring some of your new favourites Karl - industrial onion bombs !!