The other approach is to not bother with a flat base! Yes, I know it sounds daft, and it won't work unless you have a laser that does horizontal beam, ideally mounted on a tripod.
Anyway, if you are so lucky as to have one, or can borrow one, then you can use it to good advantage in 2 ways. The first is to only bother to level up the bits you are actually working on, the chassis members themselves. Use the level ot project a horizontal beam across the board, hold a rule upright so it hits the beam, and find the highest spot (where the measurement on the rule is shortest, and mark it. Then any chassis part that rests on that spot is ok, and you just wedge up the other parts until they are level.
I have 2 RSJs that rest parallel to each other on blocks, they are levelled with the laser (they are not very level, because RSJ isn't perfectly straight).
The I have several lengths of decently straight 40x40x3 box which lie across like a ladder. Each of these is wedged up until perfectly level (usng the laser), and clamped in that postion.
Then all I have to do is to clamp each chassis rail to the cross rails, and we are taliking skooker table degrees of flatness.
If the laser does a vertical line as well (most do) then you can use that to project a centreline along the jig. Just fond the centre of each tube that goes across the chassis, get the laser shining along the centreline of the jig, then centre up each cross rail. Especially useful on the upper tubes as it iwill keep everything dead square, that way you know all your uprights have stayed vertical.
I'll post a pic if I can find one
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