I was about to post my Seville cart and spotted your post. Also found it on Amazon ($74.00) heavy duty, two locking casters. My error was that I have a model 2, and in checking the width I neglected to factor the flanges that the casters on the smoker mount to. And they extended enough that my model would not fit On the top shelf of the cart.
Put on my thinking cap and left the casters on the smoker, allowing it to fit on the cart. For stability I removed the two small hex bolts on the interior of each caster and replaced with 5" hex bolts with large washers. All four legs are now securely fastened to the top shelf.
Really convenient to have everything in one spot on the cart.