UnlikeAnyOther

If you are looking for an alternative way of filling your DEF tank, I have been using this 12L plastic (Beer Fermenting) container for refilling my DEF tank. It works well, clean and easy to use. Sits right on the tank and clears the trunk liner.
Pics and a link below. And no, I don't home brew, just found this online and it works well so I thought I would share.

Couple of things I have also done. When I start the fill, I loosen the top lid to speed the filling process. I also added some clear hose to the spout and set it's length to just under the max fill level in the tank. This way when I no longer here any bubbling as it is filling, I know it's full. I also added a safety wire to the hose because when you lift the tank to get the last bit out, the hose may pop off and fall into the tank (#NotGood) so be sure to provide a secondary measure to secure the hose if you go this route.

Using this method, I have been able to fill the tank quickly and easily. The DEF tank in the E250 I believe is 26L and when the first warning "Check Additive" comes on, my tank takes about 22L to fill.

Link to 12L Tank

UnlikeAnyOther

Also, there is an inexpensive MB tool that your local dealer can order that is a good backup. I carry one of these in the trunk if I travel long distances incase the light comes on. I can easily stop at the dealer and grab a 10L jug of DEF and fill quickly without any mess. Each end of this tool is designed for a perfect fit onto the OEM 10L DEF jug and onto the vehicle's DEF tank. I know that DEF is cheaper other places (can't swing a dead cat without hitting a forum about that), but if I need a quick 10L on the road and don't want to make a mess, this solution is ideal. Second hose inside the main hose lets air out of the tank as it fills.

MSRP at my local dealer was under $20 for the tool itself. (and your dealer's price may very).
A000-583-22-04