Goe bezig walter! 
I am drooling. Very nice build!
Zeker en vast! 
I don’t know where he got his, but I found them at Cloudray laser. You can get them custom cut for any size you need there, but they also offer all the typical sizes such as 4060 , 1390, etc. I had them cut me a 1200x900mm bed for my build which cost me a little over $200 US, unfortunately the shipping was slightly more than the actual bed it’s self, so I got it for around $450 give or take a little. Still not to bad considering I couldn’t find them anywhere else for any where near that cost here in the states.
A cost efficient alternative to these galvanized steal units are alloy drop ceiling light defusers. The down side is they are not as thick and ridged as the Cloudray units, nor are they outer framed and can warp easily if you don’t have a proper under structure.
But they do work very well and you can frame them yourself with c channel alloy if you really needed to. I used them for over 4 years before I broke down and finally bought the cloudray unit. And to be honest, the only reason I make the change was because my bed is much larger than a single drop panel covered, which caused a lot of wasted space.
Here is are two links for you.
Cloudray Laser Beds
Alloy drop ceiling diffuser panels
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Thank you very much.
That is EPIC. Congrats man i absolutely love it 
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