Self-built pet cages aren't difficult to create. I built a play pen/cage for my guinea pigs a couple of years back using mesh wire, zip ties and any type of material you could use as walls for the cage. I used metal wall grids bought in Daiso, which costs RM5 per grid. Firstly, determine how large you'd like the cage to be and make sure you've considered the space for your pet's feeding area (food bowl and water bowl/bottle), play area and sleeping area. Use the zip ties to keep the cage together.
I created a spacious play pen with two shaded areas on each side because guinea pigs like to hide and the feeding and sleeping area at the upper floor to avoid a mess at the lower floor. Mesh wires are used as the base of the cage and a large tray at the bottom of the mesh wire to make cleaning up easier for pet owners. I use hydrophobic cloths at the top floor to prevent a mess from happening when my pets feed.
I also included a wooden house at the top floor. My pets loved eating the grass sold at pet stores so I bought a hanging stationery holder (it was made of small metal grids), hooked it to the walls of the cage and placed the grass into the holder to make feeding simpler. All of this saved me approximately RM100 on a pet cage and I can customise the cage anytime I wish.