Hi,
Have just finished making my 3rd DIY cage..... for the base I used MDF, which can either be painted with plastikote, or covered with fablon to waterproof it, the edges I used pine strip wood from B&Q. All of it was 12mm thickness, which made for a pretty robust structure. To prevent chewing on any exposed edges, a cheaper alternative is galvanised strapping which you can put along the edges.
https://www.toolstation.com/shop/Con...Banding/p60768