What are some methods you use to attach tanks to diorama bases? Is epoxy the only way to go?

Gday snag
Depending on the base, you can use a wood screw from under, and drive it into the bottom of the tank.
Good~ Ain't gonna move. Ever. (But if you happened to use a picture frame and left the glass in, DON"T screw it;)
Bad~ Can possibly be seen from a low angle unless covered up with groundwork etc.
You can use white glue (wood, PVA) on the tracks to anchor it also.
HTH
Cheers
Brad

I like to use white glue, it will hold well and doesn't damage paint nor plastic. Also if you ever want to remove your model it will come off easily.
I have in the past and water cleans up any residue on the tracks. It may not hold if you turn it upside down.

Tom

Wood glue ---mix mine with ground work around tracks ---have used "railway track-pins"through track --use long punch to inside of track
--one I did as a presentation piece is still locked on after 10yrs
Screws ---ok if good ground work under belly--remember and countersink base or you get in "soapy -bubble"with scratches --also helps to give "weight "to model dragging chassi down
aye
BARV
(GOSMG)

I use the screw to the base method. While screwing this I'm fixing the tank and the screw with epoxy adhesive.

I too use the screw method. I drill a hole through the bottom of the tank, remove the turret, and then (using a washer) screw the tank down into the base.

Using a screw, like this, allows you to place some tension on the tank to make it set more 'heavily' on the base.

You can pre-paint the screw black to make it less vivible if someone looks at it from the wrong angle.