We had a foal and mum travelled in our rice tripple. Take the partition out, tie the mare, but leave the foal free to move about, but make sure the trailer has doors above the ramp and close them.
Hope it all goes well for you.
Yes second what AlliJ says. Tie Mare up & leave foal loose with no partition. I have the older style trailer with a single breech bar at the front which I leave in place. Have travelled very young foals and older foals this way in an IW 505 with no problems (Even with a foal that decided to lie down !!)
Just have the central partition out, use a full breast bar, cross tie the mare but leave the foal loose (take it's headcollar off too, just in case!) We always put something against the jockey door too like a half bale of straw/hay, so they don't fall against the door but if you're talking about Archie I imagine he'll be too big and will barely get under the breast bar! When is the dirty deed happening?!