The Learning Tower your Dad made looks fabulous! I like it much better than many of the DIY plans I have found anywhere online. Does your Dad have plans that he made that can be posted?
I have actually been using three kitchen chairs with backs. I block my son in my all sides, so long as it is on an area rug, the chairs do not slide.
The tower is great though!! Kudos to you and the builder.
I saw plans for one on this website once. http://ana-white.com/ I have not made any of her stuff but I've perused her site and think she's awesome. we ended up getting one secondhand. good luck!!
Small world. We just printed it and DH went over to ask grandpa if Santa could fit one on his sleigh ;-).