Hi, I personally would not get an ovo. Re tunnelling have you thought of something like the extendible tubes like these
https://www.amazon.co.uk/Snuggle-Saf...y&sr=8-1-fkmr0
You can use them on the floor or hang from ceiling, if room permits. You just puncture 2 holes at either end and tie with some sisal or jute or other hamster safe string. You can make them small or long and bend them.
Or a whicker tunel like these
https://www.amazon.co.uk/Trixie-Wick...gateway&sr=8-1
again you can hang from the ceiling, or from floor to ceiling.
Although I think you are trying to extend to make a bigger cage. You say you have about £50 saved. Have you thought of trading up for a bigger cage zooplus do the barney for £59
https://www.zooplus.co.uk/shop/small...r_cages/189303
you could then sell your current cage, I am only suggesting this as a possibility as I think you are implying you want more space.