I've seen ramps that bolt to the side of a cage. You can get them made in almost any length.
Wooden Ladder / Ramp 150mm - 300mm Long x 94mm Wide - Cage Fixing Design - Pine | eBay
I've also taken the hooks off the long Trixie bridges and used them to hang the bridge against the side of the cage. You just make a small dent in the wood with an awl (or a small pair of scissors if you don't mind blunting them) and the wood is quite soft so the hook twists in easily. The bridge is reasonably secure like this.