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Get the Kinesis Safari Sack, buckled strap to loop around the rail on your Toyota LC so it doesn't fall off and two side straps to keep the 'A' shape for support. Without the bag you won't get sharp images.
Don't worry about hauling fill for it until you get in country and rice works just fine too. You will need @ 4.5 kilos of rice to get the right fill ratio, too much and the bag won't conform, too little and it doesn't offer support. You can buy a 5 kilo bag and give the remainder away. Also your guide will take you to a local store to buy the rice and will gladly take it off you hands when you head home.
BTW, been to Kenya and Tanzania a few times and dust isn't an issue if you use a bit of common sense. Take a microfiber cloth, face cloth type, to do a wet wipe of the outer body and lens after each days safari. Take proper lens cleaning tools and fluids you need to do a clean everyday.
As far as memory card holder, nah, get some sandwich freezer Zip-Lock bags and removable sticky orange dots from a stationary store. Full cards go in one zip bag and gets its own orange dot so you don't try to load it in the camera twice. Pretty much all cards come in a small plastic case, use these for protection inside the zip bag. Almost all card storage containers won't let you use this case and a zip bag is cheap and water tight, take a few spares.