Obviously this was a quickie but also probably more accessible for anyone else in a similar predicament. You don’t really need much other than a few small hand tool, some spacer material and a rigid platform of whatever length you or your pet prefer.
To start with, just get one of those folding utility carts. Their pretty readily available and affordable, they’re just not long enough for a dog this large.
With this design, you kind of give up most of the collapsability but that’s not a feature we often use anyway so no big deal. I will note that an improvement I have not (yet) made to this design was relocating the fixed rear wheels to be more outboard to the cart. As it stands, they’re welded in place at least 2″ closer to center than necessary- I presume this was for clearance reasons to accommodate their folding agenda, but the consequence is that the cart is much less stable as a result. If you’re aware of this it’s manageable but if you’re expecting a logically balanced cart it can surprise you. The relatively high center of gravity exacerbates this problem.
By connecting the rigid platform between the rear axle and one expanding strut it makes the whole design unable to fold. If you wanted to maintain that, you could consider using rivnuts or some other more temporary solution.
That’s pretty much it. Wyatt tolerates it, but he still thinks he’s the 3 legged marlboro man and so needs no such assistance.