Here’s a ghetto fan for you and cost me virtually nothing but a bit of time and $5 for a 4 - 5" adapter to make it’s 5" blades feed into the 4" pipe that was already installed in a cold room underground that the previous owner had built. There’s another 4" pipe that feeds down to floor level on the other side of the room for an intake. I just ripped out all the shelves and started growing pot in there 16 years ago.
Took it out of a dead microwave we were tossing out and used a couple corner brackets to make a mount for it. Used it for 10 years before finally punching a 6" hole and getting a 6" inline fan. The 4" couldn’t handle it when running 3 x 400W HIDs or the 1000W OK during the winter.
Or a DIY still for cooking up solvents for reuse. The condenser is a coffee can with copper tubing wound around a 2L pop bottle. Have a few of those in the freezer. Salt added so the water is real cold when it freezes. Pure auto anti-freeze in the coffee can for superior heat conduction and also to prevent the coffee can from rusting out like the first one did.
A DWC chiller made out of an old water cooler and a $15 fountain pump. Already had a big roll of 3/8" tubing so saved $20. Pump sits in the tub with a big coil of the tubing rolled up in the cooler tank full of anti-freeze. Nutes just circulate up to the coil then back to the tub. That thing could cool a good sized rez too.
Window screen hot-glued over the pump intake to keep roots out.
DIY bracket to hold a mogul socket on the intake side of an air-cooled hood so I could mount a 2nd light in it.
When you’re too broke to buy it. Make it!