I don't have a rasbery or an LTR/XTR, but with my cheap Huawei phone placed on the Kinoptik finder on my Eclair ACL I get a usable image with small bit of zooming (with the finger tips).
If it's just to frame, while holding the camera at arms length etc, the resolution may not matter. Screen brightness and contrast may matter much more. Establishing the ideal position of the camera (phone) may take some experimentation. Probably as close as possible. If close focus is an issue, one can get cheap plastic reading glasses with the diopter you need, even double them up. Cutting a suitable sized lens to fit. I've done that, works OK.
EDIT: The mounting of the camera (phone) onto the eyepiece. If you remove the rubber eye cup, can you grab onto the little flange that retains that. Machine something out of acetal.
Edited by Gregg MacPherson, 17 July 2018 - 11:32 PM.