I have a crude method but it works for small scale work.

I square the long edge and short edge using a handplane. Factory edges are mostly square so it takes very little work.

After that I use those two edges for all cuts, as rip cuts. After cuts I square any minor errors using handplane. Anything less than 16", I use a sledge.

If a project will require too many of these large crosscuts, I will buy a track saw.