Thanks Steve. Tipping is one my concerns too but obviously something that could be an issue with their design as well. Even as it is currently drawn, It seems to me that the even with a high center of gravity, it seems like it would have to tip at least 6" before it would fall over, but that is just a guess.
Having non locking casters might also help by letting it move a little instead of tipping. If I also put casters on the back, it will move even more freely. I had one iteration where I splayed the horizontal supports out a couple of inches to widen the base a little. The angles obviously complicated the joinery a little but still doable. The biggest issue was having to deal with the large back wheels from rubbing against it so I abandoned the idea. This is where LBD's outrigger idea provides inspiration. Instead of splaying the horizontal supports outward, I could simply add a block on the end that allows me to move the front casters out a little. That could give me another 3" in width but still be within the width of the back wheels. Of course another option is to simply make the entire cart a little wider simply by making the stiles longer.
Here are the dimensions of their metal cart from the manual. My design is pretty close in width but the distance between the centers of the front casters is only 22" so it would be prudent to widen the front a bit by adding some blocks.
I added a couple of blocks on the sides and also doubled the diagonal support and here is same view of mine. My design also has the casters a little further out in front compared to theirs. I did that specifically to prevent it from being tipped forward while tugging on the hoses.
The casters are centered under the outside block. I added the additional diagonal, figuring that belt and suspenders are not a bad idea. Making sure the block is well attached with screws or dowels might be good enough.
If I also lengthen the stiles by 3 inches, those front casters will be 28" apart and the back wheels will be about 31" wide.
I sure wish that they would ship the dang thing so I could get the actual measurements. It has been over a week since I placed the order and still no shipping notification.
--Nathan, TX. Hire the lazy man. He may not do as much work but that's because he will find a better way.
