Building thoughts

Hi there
I have been haphazardly tossing the idea around about building my own printer or
seriously modding one that I have currently.
One item I am interested in is the aluminum sheets used for the bed.
I have had a recent bad experience with going with one of those extender upgrade kits
in that the aluminum was just horrible when it began to heat up, it would warp
so bad even the poor mirror couldn't maintain a trustworthy position and to add I use
metal tape on 3 areas to secure the mirror...the warps were so bad it pulled the tape free.
I have never seen anything like it....
So my 1st pursuit is to find a dependable machined and heat treated sheet that
will provide the least amount of warping, I get it, thin aluminum will warp some
but it is typically something we can adjust for however in the above mentioned scenario...not so much.
My idea is to have a build volume of 300mm x 300mm or larger.
Any suggestions as to a resource for 3D Printer worthy aluminum sheets?
Thanks

It has become standard practice among higher end 3D printers to use a 6-10mm sheet of cast aluminum tooling plate (goes by various trade names like ATP and MIC-6) as a bed-- quite flat, and doesn't warp much at all under elevated temps. The material can be purchased from most metal suppliers, and some vendors now also sell pre-machined beds. Lots of info/details to be found on these forums and elsewhere.

If you are going to use any kind of mesh bed leveling or spring-steel build plate I'd skip the expense of the tooling plate and just go with borosilicate glass. that tooling plate flatness won't do you much good with multiple layers of magnets and metal which by-design aren't flat.

But that is just my opinion. Cast tooling plate certainly won't hurt, I just think it becomes an unnecessary expense when layering a bunch of non-toleranced stuff over top of it, but I see people do it all the time regardless.