I like gas springs because they provide a constant force throughout their range, i.e a 30lb gas springs will supply 30lbs of force no matter where it's at in it's compression. Compare that to a regular spring, which changes the force delivered based on where in it's extension or compression it is.
If you have a well setup machine shop at your disposal, check out the Sayber OSG:
http://sayberosg.com/forum/index.php
Free plans for a pretty solid machine meant to be cut from aluminium plate, it's what I would've gone with if I could've had the parts cut cheap