Cap off outlet pipe to toilet

Is this the sort of thing you are after OP?

Just be careful not to drop it down the drain!!!

Might depend on the quality? The new ones are many stamped bits of steel but I used to have some that were solid brass apart from the rubber seal. In fact I bought one cheap one just for the seal to put on an old brass one although even the older rubber seals were better much fatter allowing for more compression.

While there is nothing I like to talk more about than drains back to the OP. I suggest changing the pipe adapter that connects the toilet to the pan when the toilet is put back. I'd use something like this (only using Amazon as its easy to link to). The seals tend to loose flexibility over time so don't always create a perfect seal when reused.

If he does that straight away then this will fit as a blanking plug

Both should be available in any builders merchants.