Building a simple wood frame

[MJ76]

Hi folks, hoping someone is able to help me with what is probably relatively simple. Im inexperienced at carpentry / joinery so looking for the most straightforward option, that said I do have a mitre saw at my disposal. I am attempting to buid a basic frame which will end up being a roof for a log store which will be covered in felt. I plan on building a rectangular frame consisting of 4 peices of wood, with 2 further lengths of wood in the middle to add extra support / sturdyness, which will run parrallel to the end peices. My main question is, when I have made frames I always seem to end up with an issue where the frame 'flexes' (even when screwed together). I am using butt joint method of fixing it together which I know is not the strongest however should I be adding anything to keep the frame 'square' (like peices of wood in each corner?). Thank you =)