Sorry, but I don't know the Duplast design specifically. However, what i
have seen with other designs is to split a length of soft rubber/silicone
tubing along its length, and pushing this around the edge of the lid. This
will then yield a little when tightened against the body, and seal well.
My SpiritsUnlimited reflux still has an aluminium head, which just basically
pushes firmly into the bucket that forms the body. It's fine there with
sugar/water washes, however I've found that i need to strap it down with a
rubber bungy/octopus strap (insert local name here - the rubber strap that
you use to lash bits to the back of your motorcycle) when doing malt/beer
style washes that boil up a lot more vigourously.
Do you have enough head room in your still - is just the boiling action
dislodging the head ? If so, try leaving more headspace.