Use this tool to help hold your upper receiver open just above your lower receiver for cleaning, maintenance, or tinkering on your AR. 

After pulling out your takedown pin, swing your upper receiver away from the lower receiver just enough to slide the pin portion of the tool into the takedown lug in your upper receiver. Next, place the lug portion of the tool into the lower receiver and slide the takedown pin back into place through the tool. You should now have enough room to use a barrel rod, barrel snake, or chamber brush into your upper receiver and barrel. 

I used PETG, 60% Gyroid infill, a brim to help with warping and build plate adhesion, and tree supports for the pin and hole in the lug.