Pre-planning isn't really needed as it is very common. The material that generally is found in a cart is Fluval (yes the aquarium stuff) or 100% Pollyfill (the stuffing for pillows). There are also a number of
mods for filler replacements. Some people use blue foam (again from aquariums, foam plugs (my favorite cart
mod- by Hiighping), or pyramid tea bags (with or without straws). There are quite a few more. Each has advantages and disadvantages.
If you check in the forums under Tips & Tricks: sub Cartridge mods you will find more than you want. Try a few. Everyone has there favorite. As I mentioned I love the Highping Foam Plug-- thanks Highping!
PS Welcome to ECF!