Have been trying to figure out a way to build our own also! Here are a couple of ideas we have had but have NOT either, just brainstorms:
1. Buy a sturdy shelf made of wood or metal, the type you would use to store things in garage or panrty, they have 4 "posts" with several adjustable shelves. The kind we see are mostly metal but some are heavy duty plastic. assemble it and then flip it upside down so that the top most shelf now rests on the floor, find a way to make the bottom very stable ( ie: nail together a base made of 2X4s into a "frame" that would give the bottom extra surface area so tipping was less likely).
Set one of the shelves to the level we want it to be at so our child can reach the counter is standing on it. Reinforce it's safety by nailing 2x4s, vertically between the shelf for stading on and the wood frame we made at the bottom. The 2x4's would be cut so that they had to be hammeded into place and would very tightly wedge tightly between the shelf where you want child to stand and to the wood "frame" we constructed on the bottom of the shelf for stability (as mentioned above) and would be nailed or scrwed into place for extra security. So basically taking the store bought shelf as the skeleton and adding lumber to make it secure and safe so that the shlef could not drop from it's brakets and so that it would be less likely to tip over. Ya never know, this one could work!!!
2. Buy an actual expensive name brand learning tower with the sole purpose to trace a pattern from it's pieces and never assemble it. Once it arrives from whereever you bought it, unassembled of course caus that is the world we live in, trace all of the main wood pieces onto large sheets of paper and take photos of any part of the assembly that you are unsure of. Put all of the tower pieces back into the box, totally unharmed, and send the tower back for a refund. Take the pattern and pictures and build our own tower using the "knowledge we gained from seeing the tower pieces" and using mdf board and lumber that we buy at the hardware store or score from a friend, freecycle, etc. This one is a bit shady I admit, and you might end up having to pay return shipping, but I am not planning to use the pattern for selling and those towers are so darn expensive! The tower would be returned in perfecrt condition and would never even have been assembled. Just an idea.....but if anyoen tries it, let me know if it wokrd out so I can do it too!