I have been using a very simple router sled.
It is really simple, convenient and cheap.
You just need only 4 linear rails, 8 bearing blocks.
And some bolts and plywood.
SBR 20 : 1850mm, 2pcs
SBR 20 : 1200mm, 2pcs
SBR 20UU : 8pcs
They cost USD135.21. And I needed some bolts and plywood for the setting.
I was using a 20mm router bit for flattening wood.
But that is a little bit small for flattening work.
So I got a new one from China.
It is great.