Links shamelessly stolen from rebbit. You can fit five per cart. If you're planning on using any of these in the future you might want to get them now before they have a pricetag slapped on them.

https://robertsspaceindustries.com/pledge/Upgrades/P-52-Merlin-To-Aurora-ES-Upgrade
https://robertsspaceindustries.com/pledge/Upgrades/P72-Archimedes-To-Mustang-Alpha-Upgrade
https://robertsspaceindustries.com/pledge/Upgrades/Aurora-LX-To-Mustang-Alpha-Upgrade
https://robertsspaceindustries.com/pledge/Upgrades/Dragonfly-Yellowjacket-To-Aurora-LN-Upgrade
https://robertsspaceindustries.com/pledge/Upgrades/Dragonfly-Black-To-Aurora-LN-Upgrade
https://robertsspaceindustries.com/pledge/Upgrades/MPUV-Cargo-To-Aurora-LN-Upgrade
https://robertsspaceindustries.com/pledge/Upgrades/MPUV-Personnel-To-Mustang-Beta-Upgrade
https://robertsspaceindustries.com/pledge/Upgrades/85X-To-Avenger-Titan-Upgrade
https://robertsspaceindustries.com/pledge/Upgrades/300i-To-Mustang-Gamma-Upgrade
https://robertsspaceindustries.com/pledge/Upgrades/Mustang-Gamma-To-300i-Upgrade
https://robertsspaceindustries.com/pledge/Upgrades/Hull-A-To-Avenger-Upgrade
https://robertsspaceindustries.com/pledge/Upgrades/Reliant-Tana-Skirmisher-To-Reliant-Kore-Mini-Hauler-Upgrade
https://robertsspaceindustries.com/pledge/Upgrades/315p-To-Reliant-Kore-Mini-Hauler-Upgrade
https://robertsspaceindustries.com/pledge/Upgrades/Mustang-Delta-To-Reliant-Kore-Mini-Hauler-Upgrade
https://robertsspaceindustries.com/pledge/Upgrades/Mustang-Delta-To-315p-Upgrade
https://robertsspaceindustries.com/pledge/Upgrades/M50-Interceptor-To-Cutlass-Black-Upgrade
https://robertsspaceindustries.com/pledge/Upgrades/Freelancer-To-F7C-Hornet-Upgrade
https://robertsspaceindustries.com/pledge/Upgrades/Buccaneer-To-F7C-Hornet-Upgrade
https://robertsspaceindustries.com/pledge/Upgrades/Gladius-Valiant-To-F7C-Hornet-Upgrade
https://robertsspaceindustries.com/pledge/Upgrades/Gladius-Valiant-To-Freelancer-Upgrade
https://robertsspaceindustries.com/pledge/Upgrades/Buccaneer-To-Freelancer-Upgrade
https://robertsspaceindustries.com/pledge/Upgrades/F7C-Hornet-To-Freelancer-Upgrade
https://robertsspaceindustries.com/pledge/Upgrades/F7C-S-Hornet-Ghost-To-Freelancer-DUR-Upgrade
https://robertsspaceindustries.com/pledge/Upgrades/350r-To-Freelancer-DUR-Upgrade
https://robertsspaceindustries.com/pledge/Upgrades/350r-To-F7C-S-Hornet-Ghost-Upgrade
https://robertsspaceindustries.com/pledge/Upgrades/Freelancer-DUR-To-F7C-S-Hornet-Ghost-Upgrade
https://robertsspaceindustries.com/pledge/Upgrades/F7C-R-Hornet-Tracker-To-Freelancer-MAX-Upgrade
https://robertsspaceindustries.com/pledge/Upgrades/Prospector-To-Freelancer-MAX-Upgrade
https://robertsspaceindustries.com/pledge/Upgrades/Prospector-To-F7C-R-Hornet-Tracker-Upgrade
https://robertsspaceindustries.com/pledge/Upgrades/Freelancer-MAX-To-F7C-R-Hornet-Tracker-Upgrade
https://robertsspaceindustries.com/pledge/Upgrades/Retaliator-Base-To-Constellation-Taurus-Upgrade
https://robertsspaceindustries.com/pledge/Upgrades/Cutlass-Blue-To-Constellation-Taurus-Upgrade
https://robertsspaceindustries.com/pledge/Upgrades/Vanguard-Hoplite-To-Constellation-Andromeda-Upgrade
https://robertsspaceindustries.com/pledge/Upgrades/Esperia-Blade-To-Redeemer-Upgrade
https://robertsspaceindustries.com/pledge/Upgrades/Vanguard-Warden-To-Redeemer-Upgrade
https://robertsspaceindustries.com/pledge/Upgrades/Retaliator-Bomber-To-Constellation-Aquila-Upgrade