This design has an unstable mode that folks should be aware of.  Fortunately, it isn't very difficult to eliminate.  To be stable you always want the load to be distributed to the two feet on each end of the stand.  With the current design, this indeed occurs while the telescoping section is pinned.  The load is distributed out the four legs (upper right sketch).  However, while bottle jacks are used to raise the plane, the load path is no longer through the legs but instead from the plane straight down through the bottle jack to the floor (upper left sketch).  This isn't too bad if only one side at a time is jacked up. Some fore/aft stability is provided by the other side and/or the wheels still on the ground.  It gets a little dicey when both left and right side are being hoisted at the same time, the plane could fall forward or aft because none of the four jack stand legs are supporting the plane.  One could theoretically crawl under the plane and lift the stand right off the floor.  To be stable while hoisting the plane, the forces need a path through the structure of the stand (lower center sketch).  Placing the bottle jack on an additional beam between the two legs provides this path.  

Chris,

 

Egads, I have constructed a similar jack, but a little bit different.  I used 2 inch square steel tubing and 1.5 inch square tubing for the sliders in the vertical tubing pieces (overbuilt, just like my Lancair).  There is an area washer welded to the top of the slider for a bigger base that the wing rests upon with the jack locating "knob" fitting in the washer hole.  Also, there is a thin steel plate welded between the feet on each side and electric scissor screw jacks (12 VDC, $50 each) are machine screwed to those sheets.  Since I only use the jack for my plane, the cross piece, connecting the two sides, is bolted in place.  This jack is rigid in every direction.  I still have the jigs for the cutting and welding up such a jack (already used by another to construct one of their own.  I searched my computer for pix, but could not find them so I will take a few and send them out along with a parts list.  Each side is infinitely adjustable thus allowing for lateral leveling - all done with push button ease.  BTW, it is painted with old out-of-date two-part tank sealer.

 

I considered putting the jack on wheels, but the smarter side of my brain said, "No!"