I used to make mine from old off-cuts of fleece or charity shop baby clothes!
The sleeve off a fleece top can be cut off and straps sewn onto the ends to make a fleecy tunnel.
Inside a tank though, you might need to put a shelf inside for him to be able to reach a high hanging hammock or tunnel. Mice do like to climb, but try and keep things easy for him to get in and out of easily. A simple wooden half shelf made from an old pine shelf and a ramp leading up to it would be good. Legs for the shelf can be made from blocks of wood and nailed in each corner. Or you could use large fiddlestix ladders!