I use Shapeways for printing my models and the their versatile plastic material. Its nylon plastic (PA12, Polyamide). I know Shapeways is on the pricier side but I think its worth it. It's professionally made and its SLS printers instead of FDM printers.
https://www.youtube.com/watch?time_continue=37&v=9E5MfBAV_tA
"Versatile Plastic is a durable nylon plastic that can be used for a wide range of applications, both for prototyping and for end products. Printed using Selective Laser Sintering (SLS), when thin, it's flexible enough for hinges and springs and when thick, it's strong enough for structural components." (Shapeways.com)
I find its very rigid but it is plastic and there is some play. Less play then I was expecting though. Some of the play can be removed by increasing the thickness of the bars or putting "pillows" on the eye peice like in the picture. Ideally youd fasten the pillows around the eye piece to for extra rigidity.
I use a 3,7V 2000mAh Li-Ion battery and a 3,7V-to-5V power booster with a 1A output to power the Raspberry Pi 3 and the screen. Ive tested a 16mm lens to but the image got to small for scopes but for red dot sights I found that 16mm is the right one. But that only applys to the official Raspberry Pi camera. Different cameras have different size sensor chips.
How are you going to aim if you dont use a scope? Are you going to use a red dot sight or maybe a laser sight?
Can you post a picture of your setup?