Open hole of wires

I wouldn't just cover it. Please get a competent registered electrician to check the property first to make sure everything is safe.

The hole/wires can be concealed with any suitable material wood and plaster included. However the cables must be either >50mm deep from either surface or in so called safe zones which is directly up/down/left or right of a wiring accessory such as a socket or switch or the new consumer unit for example - for more detail google ‘wiring safe zones.’

It is safe as long as it's all dead. I'm assuming it's an old fuseboard location with old dead wiring left from a rewire? It may be worth getting an electrician out to verify it's all dead before you proceed with covering it up.

Look for sockets and lights around so you avoid covering up feeds , so to replace legs from the board, joint boxes are to avoid ,