Tonight, I imposed upon friends who already do SO much for me and my dogs. I loaded all the pieces and parts in the van and motored over to visit Don and Jacque. Don did all the fine-tuning on cutting the pieces to fit exactly in the xpen and attaching the "L" trim to the two side pieces. Jacque and I, uh, "supervised" the operation. I think Don invited us more than once that we could go back in the house, he'd manage without us. :-P
Now, it'll take a bit more paint and cleaning up, and I have another piece of tileboard to attach to the swinging door to make the wall appear solid where the cut-out is (when the door is closed), but voila, puppy pen for under $25, it fits perfectly inside the small white pen, and it is completely flat for storage. whew! Oh, and with one 4 x 8 sheet of tileboard for each box, I got the 4 sides, plus half of the sheet to put under the entire thing. Shiny white and completely washable.
Some pix of the puppy pen. As Garrett would say, you should be able to click and "biggify" the photos.