Heya
normally for pet class people use plastic carriers, if it is a busy show with lots of enterys if everyone brought big cages along it soon fills the table and no one can get their hamster safely on. I would not use a card carrier at all, I have had escapee's on the way home from shops only 10 mins away and an hour getting them out the car again!
Idealy a little cage, something like a combi 1, a creti 9, or an ovo dome (i think its the gym I am thinking of) are good 'small' cages, altho some people bring mini duna's which are a bit big and if the level is in then its hard for the judge.
the above cages are often sold on ebay and gum tree pre-owned, normally very cheep and sometimes free, so if you keep your eyes open im sure you can find something. The carriers I use are about £10-15
You do get points on travel conditions, like having a dog biscuit or a wooden chew, itl give the hamster something to do when out, having a small tunnel you can remove when traveling, or a hanging kabob toy are ideal for pet class, however I have seen many bring syrians with mini duna wheels, fluffy bedding ect that ofc will take away points because they are not safe (and the judge will often make a side note to tell the person if its not a hamster suitable like fluffy bedding)
A water source, I use cucumber because its mostly liquid, refreshing and lasts the day, Others bring small 'mouse' size bottles to add in, remember when in cars or trains you should remove bottles, tunnels and kabobs because they can hurt themselves on it whilst traveling.
make sure theres food in there, if you want to add extra's like millet sprays, wheatabix, hanging chews ect, and some bedding, and substrate,
normally judging is at 2pm, altho i would try and get them benched by 12
I hope you can make a show and best of luck!