So Tenser's floating disc "follows you so that it remains within 20 ft" if you are more than 20 ft away.
So let's get a bit of carpentry skill together, and (whilst keeping the total weight under 500-1000 pounds) rigging up a cart that is 25 feet long. The underside of the carriage is 3 ft tall, and the wheels are 3 ft diameter.
The wizard casts Tenser's Floating Disc, and the party's barbarian picks up the carriage, and puts the rear end on top of the floating disc (let's strap it in with some ropes for good measure).
The wizard then climbs on the carriage, and moves towards the front.
As soon as he gets > 20 feet away, what happens? What speed does the happening occur at?
Bonus question: Assuming the above answer is that the carriage moves, at a high enough speed to be useful, any ideas how to steer such a device other than the wizard moving back within 20 feet to stop it and turning it manually like a peasant?
To alleviate concerns that the carriage is an obstacle to the disc, an alternate construction could instead use a 50 ft plank, not touching the ground, with the disk in the middle, the wizard at the front and an equal weight counter balance on the other end. That way, the disk is carrying things, but it not touching anything other than the things it is carrying.
Best Answer
Nothing Happens
From the PHB, page 282
Given that it cannot move to within 20 feet of you, it won't move. It would consider the cart as an obstacle.
Your best bet for your "tenser's physics defying discmobile" would be for the wizard to get on a horse and let the disk follow. Everybody else can sit on the disc.
But if it does some how work, and you need to steer...
Let the barbarian hold the wizard out on a stick 25 feet long. He can steer the cart by changing where he hangs the wizard from. You may need a counterweight so he doesn't get tired.