If you want to make the height adjustable I?d suggest bracing the upper and lower legs individually and using a scissor jack or bottle jack mounted between the two to adjust it up or down. You?ll obviously probably want to pin the legs once you get it where you want it as I wouldn?t rely solely on the jack to maintain position.

I?d likely just scavenge a small scissor jack from the local auto salvage yard. Probably cost you $5.