oldandslow
Kennym made my sleds
I made some short ones but it can be done on longer.
for a .001 sled 36" long, use two 3/4" x 37" long boards, sand both flat as you can, set one on top of the other, mark a line at 36" you need .036 thick shim between the boards at that line, at 18" you need .018 shim , at 9" you need .009 shim, at 27" you need .027 shim, secure them together and and run it threw a thickness sander of plainer. take it off and measure the thickness every 1" and see how good it is.
If there is some way you can get that done on a .001 and .002