Hi Kia ............ the simplest option is to buy a 24" standard hardboard flush door. (That's a smooth faced one with no fancy panels.)
That will measure 610mm. Rip one door of the right width ........ then trim the remaining piece to arrive at the 302mm you need.
If you can't find a 24" then get a 27" or a 30" and do the same.
You'll need to rip or plane an infill piece of softwood to make a stile for each door (filling the open edge as you say)
Same applies to cutting the door to the right length. I've done that many times. It's a simple straightforward job.
Probably best not to use a plywood door, as the ply can splinter and give a rough finish.
Good luck