Dumpsty is actually handmade by another company, we just thought it was cool so we asked if we could sell it for them. We looked into steel beams but it was too expensive to sell. Cold weather is the bane of cement, so Q4 is the only time where we get close to running out of product. The great thing is that we make everything ourself, so we just make more.
There are a couple of people copying it. The main barrier to entry is that you need to do volume to make money, and to get to that point you need to do everything yourself or you'll be spending too much out of pocket. My partner and I can create products, design, build sites, market and advertise, run warehouses, do our own accounting, etc... We haven't hired outside help for any of it.
You've hit the nail on the head. There's a LOT that goes into something like this. It's hard to replicate, and even if you did, the margins aren't there to undercut much.
Founder of Mini Materials here. I'm a designer myself, and I absolutely hate e-mail popups. BUT - it just works. The wheel captures a huge percentage of emails and that leads directly to sales.