Like Mark I'm a curious/dubious of the low rider style - it seems like it could end up with some awkward angles both up high and down low ... I reckon a good adjustable chair, or a little step might be a better solution. An 18" shouldn't need a ladder, just one step.
Check out these babies -
http://www.catseyecollimation.com/cperch1.html
http://www.catseyecollimation.com/summitwscope-b.jpg