Large Trofast flisat lid with a cut out section to fit a 25.5cm lego base plate I have used a univeral plate that does claim to fit all major brands including LEGO, DUPLO and more (The fit is 32x32 studs)
 
You get a plain 3mm large insert, and a 3mm hollow one with a 25.5cm square hole which will be glued to the base. 
You then simply sit your base plate in the space. This won't be glued in, as you may want to change the base plate colour for different themes So please select from the menu whether you want a plain lid with no base plate (ie if you have your own of the same size) Or select from the colours in the menu PLEASE NOTE : IF YOU ARE BUYING WITHOUT THE BASE PLATE AND HAVE YOUR OWN, PLEASE MEASURE YOURS AND LET US KNOW IN THE COMMENTS BOX IF YOU NEED US TO ADJUST THE SIZE (We can't go larger than 26cm as it won't fit on the insert)