I’ve had great success with Aluminite Casting resin . I first bought the starter set ( about $30 Canadian ) which gives you the RTV rubber and catalyst as well as the 2 part A/B resin . It’s very simple to use as you get the exact amount of catalyst for the RTV supplied . It gets trickier when you buy the RTV in larger sizes , but with practice you get the hang of it . The A/B resin is a 1 to 1 mix , which is great , I’ve used as little as 1/2 a tea spoon to cast a box . You get 2 – 8 ounce bottles so you got lots of casting ahead of you , mine actually went bad on me ( about a 10 months ) but I cast tons of ammo boxes and gas cans 
Depending on the part your casting , you can also use plasterscene as your mold , just push the item in , pull out carefully , and add resin . I’ve done that to back packs and canteens/ breadbags for my Bolt Action Brits and Germans . Check your local craft store ( Michels ? ) for Amazing Casting Resin , which is the Aluminite stuff under a more family friendly name . They also carry a variety of rubbery type reusable mold compounds , I’ve tried a few but never seemed to get it to work for me . I also cast the Panzer IV cupola for a beute T-34
Hatch parts for a KV 1 cast using plasterscene as my mold . I used the resin that turns white when hard , but you can get IRC a couple of different types of clear resins
