There are a bunch of handlebar plugs here on Thingiverse already, and several really nice designs. (This one is particularly nice and I've printed and used it on several of my bikes. The only issue I have with most of the designs is they're heavy, or they don't fit my bars. I set out to address both of these issues, and what we have here is the culmination of several month's worth of design variations which I've been testing extensively.

The design is minimal - the outer diameter of the plug is 22.2mm (7/8") which matches the outer diameter of MTB handlebars.This allows the plugs to stay in place when grips are installed or removed. The design is sized to the inner diameter of the handlebars, which you'll need to measure. (or print several and see which fits) For instance, the 17mm size fits my ENVE bars, as well as my wife's Bontrager XXX bars. The aluminum bars that came with my XC bike has thinner walls and so they use the 19.5mm diameter size. The design is parametric, but to make life easier, I've included STL files in a range of sizes from 16.5 to 20mm. The Fusion360 F3D file is included if you wanted to make any further changes. (see the section below for notes on that)

These things are light - see the photos, a current set of ESI plugs is about 4g and these come in at 2.5g in the 20mm ID size in PLA. Yes, this is ridiculous weight weenie territory, but I'm not aware of any option that's lighter, aside from running no plugs, which is a bad idea as that's kinda dangerous. Of course, if you wanna be really fancy, you can always spend $20 on a set of carbon fiber plugs which weigh over double as much.