[alpacassei]

Woodshavings are safe in the UK, I use them for most my hammies but avoid actual sawdust since itks very dusty and could cause respiratory issues. Fitch, Carefresh, and Kaytee Clean & Cozy are all good paper substrates. Fitch is the cheapest because you can order it in bulk but Carefresh and Kaytee come in lots of different colours. Avoided any scented substrates though since hamsters have very sensitive noses. You can also use hay in areas of the cage!

For the actual nesting material, it’s best to use torn up tissue strips rather than commercial nesting materials, and avoid any kind of fluffy bedding as it can cause serious injuries.

How much smaller is the furniture? Once the substrate’s in, the cage will be pretty heavy so it doesn’t matter if it sticks out over the edge of the furniture a little bit (no more than 20cm on each side though).
You can use sisal or hemp string to hang the Stackafuzz!