Warping on large parts, Creality Ender 3

[CambridgeMart]

I'm trying to print a cover for my AC unit; I'm having to print it in 8 sections as it's too large to print in 1 piece; the problem is that it's very thin in section and I'm having problems getting all the details printed correctly and there's also a tendency for it to warp. The print does extend close to the corners of the print bed and I'm wondering if it's the temperature differential that's causing it. The print is PLA, using filament that I've successfully used on simpler models; running 45C on the bed, any lower and I lose adhesion. I've tried using support all and both raft and brim bottom layers, but with no effect on the malformed parts and warping.
I'm using the Creality Slicer that came with the Ender 3, most settings are at default except for the bed temperature.
What are the best things to try to prevent warping?
I've attached my stl file for reference