I have a daughter who may want to learn how to ride a unicycle. I saw the unicycle trainer and that’s something I have the ability to make, maybe.
Did anyone make their own?
I’m sure I can add other unicyclists to the group more easily with one of these, if it cuts down the learning curve like they say…
Always be skeptical of something that professes to help one learn how to do “A” more quickly without actually doing “A”.
How old is your daughter? I just held my two boys’ (one is 10, and the other is 8) hand and walked down a path beside them until they were comfortable letting go. It took a few days before they started letting go, but not long after that, they were launching themselves off the fence, and started making progress really quickly.
I almost don’t know if it would’ve been worth the effort building something… But if you like building things, then by all means, build away. 
I’m not sure what a unicycle trainer looks like, but here’s a slightly related story. My nieces were watching me try and ride it a few weeks ago (6 and 8 years old). I would get on, and then fall off immediately. One of them said “they should make one with a training wheel and handle bars”. The younger one said “yeah, but then it would just be a bike.”
Best way to train
I’d recommend the following:
1.) high rail(shoulder height) for raw beginners. Just learning how to pedal and stay on.
2.) low rail(hip high) like a tennis court net for more experienced beginner. Pedaling is almost natural.
3.) shopping cart for the advanced beginners. You are just about ready to be able to do it, but not quite.
Keep on…
I tried the shopping cart method one time. Some homeless person must’ve abandoned it, because it was nowhere near Ralph’s Supermarket. It felt really awkward. I gave up after a few moments. That happened, of course, after I already knew how to ride. What made it so awkward was that I sacrificed all my normal systems of balancing the unicycle while holding onto and pushing the cart. It might have worked better, but I’m on the tall side, and I felt like I was reaching down too much while holding onto the shopping cart handle. Also, the cart was not rolling that smoothly. YMMV.
Fewer crutches, more safety gear. That’s the way to go, IMHO. I held onto a fence the first few times on the unicycle. I gave that up pretty quickly. After that I only used the fence for launching, and within a couple weeks, I got tired of walking back to the launch site and learned to free-mount.
If you train yourself to keep balance by using a crutch, you are doing exactly the wrong thing. If you’re using the crutch to extend the amount of time you spend self-balancing, then that might be okay. A couple recent beginners described using ski poles to learn. Sounds like that method worked well for them.
I’d say if you are looking for an excuse to get into the garage and tinker anywhy, why not? I know I’m often looking for those. It’s probably fun for your daughter to play around with, and won’t cost that much in material. Will it help the learning curve significantly? I haven’t seen one used yet, but I’ll doubt it.
I have found that variety helps when teaching kids though, so having another method is always good. There is a lot of keeping their interest in it involved. When I gave workshops, I wouldn’t be able to teach one on one for much time, but I’ve found that riding using your hand as a support, while you are either on foot or on a unicycle works as a pretty good motivator. I think it’s getting a taste of being able to go anywhere you want on a unicycle (anywhere in the gym, atleast), that is so motivating.