hi, I have similar interests, I like making, building and designing my own things; work benches, tools, cages, shelves ect....right now my rats are all in storage tubs with wire tops. I would like to give them better places to live than that. more engaging environments were they can exercise burrow and climb, but still easy to maintain/clean.
An important thing to keep in mind is that building cages from scratch, and even modifying used cages to be suitable, adds up very quickly in cost in regards to both money and time.
Over personal observation through the years, it's VERY rarely beneficial to try to make a cage rather than seeking out a suitable cage to begin with.
If the joy of customization is your goal, consider really modular options like Martin's cages. You can order basically just a quality cube and bottom tray from them and then customize the inside with your own shelves and toys. Following Isamu's advice, I've replaced my shelves with kitten litter boxes filled with a layer of bedding. This dramatically improves odor and provides great digging opportunities. Ramps are not needed for average, healthy rats.