From this thread-
Exquisite P-bass, Unboxing and Review - PART 2
Post #150
"The first thing you need is a dead flat Sanding Beam.
NOT a Radius Sanding Beam (because these bases have a Compound Radius. 12" radius at the nut, and 16" radius at the 20th fret). A Radius Sanding Beam would undo our valuable existing Compound Radius.
Q: Vid, should I buy an expensive Sanding Beam? I want to do it **right** !
A: Hell no.
Go to your local Harbor Freight (Canadian Tire for those with health care) and find this item #69242:
It's a heavy, SOLID 24" aluminum level. Not a hollow box level, or I-beam level.
Now the reason you want to go in person to HF, is so that you can make sure it's dead flat. Put 2 of the machined sides face to face, and hold them up to the light to make sure they are dead flat to each other. If you see ANY gap, try another until you find a perfect one.
It has been my experience that 99% of these are really well machined when tested against my $300 machinist's straightedge. Flatter than a slice of Corian or marble for sure.
One is all you need, but if you plan to do fretwork all the time, get 3 so you can have one for each grit sandpaper always ready.
For $13 after coupon, you are not going to find a better Leveling Beam."
No affiliation, just sayin...