With the arrival of summer I've been finding that passive cooling with a small heatsink is inadequate for any Pi under more than a light load. I wanted a design that had a fan blowing down onto the heatsink, but didn't want to mess around with an enclosed design that would most likely need a second fan to properly exhaust air out of the case. I was also too lazy to redesign a case for the new port layout of the Raspberry Pi 4. So I took the bottom of Credo's case (https://www.thingiverse.com/thing:1368797) and added holes for mounting the Pi to it so the bottom of the board would be protected, then I took niftymaker's 30mm fan mount (https://www.thingiverse.com/thing:2755779), fixed the spacing of the fan mounting holes, and added integrated standoffs. To mount the fan I used the screws that came with the "iUniker" Raspberry Pi fans I got from Amazon. They're not the greatest fans in the world, but only cost a few dollars each, and they have the leads to connect to the GPIO pins on the Pi already installed. The fan bracket uses 20mm M2.5 screws that extend through the Pi into the base.
Even with a cheap fan, I'm getting CPU temps 20C lower at idle in a hot room vs. heatsink alone. I haven't put the Pi 4 through its paces yet, but this setup maintained the CPU of a Pi 3B+ at 55C under 100% utilization with an ambient temperature of 28C.
Edit: One serious downside of an open case design is that a curious cat might decide that it is fun to stop the fan with a claw and then bite it.
Doesn't really need 100% infill, but most of the parts are so thin that it's almost solid anyway.