I would use some of the store existing flooring and cut it to the right width (cutting the recess side, not the tongue edge), then fit it into your existing floor. The only issue then would be that the groove side(or what used to be that until it was cut off); will have no way of interlocking into the existing flooring, but I would nail it down if it was wood subfloor, or perhaps stick it down with pink grip or similar on that edge and also the edge of the floor meeting it. Yes, it is supposed to be a floating floor for expansion, but I don't think you will get any issues with it.

Failing that use a proper tile threshold to keep them together, might look nicer depending how good at DIY you are.

The row you fit won't be as wide as the other planks but it will at least easily fix this issue and when all the doors open, it will all match.

Quick note, if you are doing this, then make sure you knock all the planks together so there's no gaps(can put trainers on and kick them, or a tool if you can get to each bit) which can appear after some time. May as well use it to fix any other niggles with it .

If you don't have any spare planks, take one to your nearest hardware store and try and match their style and colour etc.