This is a dimensionally accurate dummy for a GoPro Hero 5/6/7 that you can use as a placeholder on your RC vehicles for those times you don't want to risk your actual GoPro, but still need to have the proper weight and CG for tuning or maiden flights. There is a slot inside that you can stuff with pennies to get within 1g of the actual weight of the GoPro. I used 32 or 33 pennies (I can't remember, lol). If you print with the correct infill settings, the plug should take up the remaining space in the slot and prevent the pennies from rattling around too much, keeping the weight distribution constant.
This is designed to be printed in TPU. Print the GoPro with 30% infill and print the plug with 50% infill to get the weight correct. Your results may vary depending on filament and print settings, but it should be pretty close. You can always print with higher infill and use a few less pennies to match the weight as well. I used Simplify3D's "waggle" infill setting to also make it a bit squishy to help absorb impacts on those failed wing launches.