The cheapest and easiest item to build in the Foundry is the Cipher, but it will take a fair amount of time to get to a point where you are able to build them.
First, you will need to buy the blueprint from the Market. It only costs 500 credits, so it shouldn't be too hard to acquire. The blueprint is re-usable, so you only need to buy one.
In order to build a Cipher, you will need 400 Ferrite and 400 Salvage. The Ferrite is easy to pick up as a common drop anywhere on Venus. Salvage, however, doesn't show up until you reach Mars and/or Jupiter, which can take quite a long time depending on how quickly you progress though the first few planets.
In order to build 20 Ciphers, you will need a total of 8000 Ferrite and 8000 Salvage, which may require a little grinding until you have collected enough.
Alternatively, if you don't mind spending real money on the premium currency, you can purchase the Ferrite and Salvage from the Market. Each will cost 30 Platinum for 3000 components.
Once you have enough to build a Cipher, each Cipher will cost you 100 credits and take 1 minute to complete. Simply build 20 and this trophy is yours.
If you're not in a rush to earn this trophy, I suggest building one Cipher between each mission that you play. Sitting on the Foundry screen for 20 minutes waiting for Ciphers to build can be a little boring. It will take a lot longer to earn, of course, but at least you can spend the time a little more productively, such as continuing to grind some of the weapon proficiency trophies.