I have an old Tigurius model that comes on a 25mm base - but I thought being a character he deserved more of a base. I was concerned about the rules lawyers, but between Galahad and myself we thrashed out a bit of a work-around: a double base.
Here's the prototype, which I may actually go on to refine with a bit of Greenstuff and more scenic stuff on the actual base - but here's the principle of the project.
1x piece of card
(like the back of a blister pack) - plasticard would be better.
A craft knife
Whatever basing materials you use.
Use the 25mm base to draw a circle onto the 40mm base as a cutting outline. Get the 25mm as central as possible before you draw - I recommend a bit of bluetac to hold it in place.
Use a craft knife to cut the circle out. I messed up the initial 40mm base I did, and the one above isn't pefect - it's because of the peg holes that are molded underneath the base. If you can remove these, do so. I don't think my knife was sharp enough.
Don't worry too much about neatness if you're going to add a ton of scenery.
Draw around a the 40mm base on a piece of card and cut it out. Glue to the bottom of the base.
Test fit. It might be worth, at this point, magnetising the bottom of the base to the card of the 40mm base.
Ta da! Based, ready to prime. Base however you want, but obviously ensure you can still remove the model from it's 40mm holder. I'm going to paint a light effect around the join, thus making a feature of the crack. Finer basing materials would make the join far less prominent.
Don't forget to prime the inside of the base.