Congrats on your upcoming litter & for researching on all you can do for a safe whelp for your Leia. I have had 3 litters so far (with my 4th on the way in a couple of weeks), and each time is as exciting and nerve-wracking all over again.
I see that you have decided to go with the plastic container & x-pen......this is what I was going to suggest to you. I have used an under the bed plastic tub for each of my litters and it has worked out well. It is easy to clean and when not in use, it stores blankets, towels, rugs, and such for the next time. Ive attached a link to a picture of my set up from one of the earlier pregnancies we've had.
Good luck to you and Leia.............keep us posted!
http://www.yorkietalk.com/forums/2748894-post201.html