The i3 Plus has passively cooled electronics, which isn't great when using an enclosure. This "Thing" adds two fans to the table the printer sits on and neoprene air guides so that cool air can be flowed though the enclosure.
My exhaust air is usually around 22C with enclosure at 35C+ and 20C ambient.
I use a CCTV style 12V PSU and a cheap PWM fan speed controller all from ebay. I also use a CCTV power splitter and a cheap 12V LED remote to control my enclosure lights from the same PSU.
I've included a picture showing the parts. Note that I usually solder on plugs and sockets, but the LED dimmer module has some screw terminal ones, again from ebay, just search for 12V CCTV plugs and sockets. A "wall wart" like shown will drive fans and a few LEDs, but I've got a lot more LEDs so use an LED lighting transformer.
This uses the Plus brackets on the top table but Lack stackers without extra height lower down. I've also added other Things from all over the place and designed my own cooling system using a duct and some 25mm wide neoprene strip. The clamps keep the neoprene in shape and the corner guides go at the back corners of the printer to ensure I put it back over the cooling in the right place.
Filament pass through. http://www.thingiverse.com/thing:2027206
Fan filter. http://www.thingiverse.com/thing:1284759
Fan grille. http://www.thingiverse.com/thing:1879694
Lower table stackers: http://www.thingiverse.com/thing:954332
New magnetless stackers: https://www.thingiverse.com/thing:2226678
Regards the magnetless stackers, those aren't what I used, but the original "thing 1843235" has gone away. I have included the lack_foot and lack_foot_top stls and scad files as the licence was Creative Commons Public Domain. You can either use these stls or take a look at the magnet remix.
You'll have to do your own measuring to judge where to put the cooling if you decide to use it. Note that my fans are doing a great job so going smaller than 80mm is definitely an option!
Note that the neoprene clamps didn't quite work as designed and the inner parts are probably all you need. I can upload the STL of just these but the OpenSCAD file is there if people want to play around.
The hinge spacers could easily be added directly to the hinge (import into OpenSCAD of spacer and jiggle around) but I'd already printed the hinges when I spotted a spacer was required with this panel mounting system.