We had the Upside-Down bed from IKEA for our little guy, but my husband continued to complain on how he had to "crawl" into the bed. Okay, scrap that. No problem. I just searched Ana's plans for a nice bed I could make. And add a trundle. There you go.
I did modify the height of the headboard, as I plan on moving Jack to an upstairs bedrooms that has slanted walls. I wanted this bed to fit in any configuration, so I had to take 10" off the headboard. Also, due to the impending moving, I wanted the bed to easily be taken apart. So I attached the side rails with surface mounted keyhold brackets from Rockler Woodworking. They work great.