making a door

I am going to get another box of NIC Grids so McQueen can have a 6ft x 3ft cage in the lounge. The only problem is I don’t know how to make a door out of these panels. Any suggestions?

I don’t think a pen would be suitable because he would just jump out of it.

You could just make a door out of however many panels you want, attach with cable ties (not too tightly, but tight enough so it doesn't sag) on one side as a hinge, and use something like a couple of bulldog clips (like this) to hold it shut on the top/other side
The other option would be to use a pen but make a lid out of the cubes (or use them to make the pen sides higher), might be a bit more sturdy if he decides to bite/tug at enclosure

I would say the cable ties too and then maybe a couple of pegs or padlocks to secure the door shut when not in use.

yeah i agree with cable ties