And in 15mm.
How wide are the trech bottoms?
Would they take figures on a base 30mm x 30mm?
In places and on the firing steps, yes. Theyare designed for figures based on pennies but the trench floor area is wider to allow for the models to be used with multipple models. Heres a 3 minute video showing them in more detail, which might help answer your question?
Here are some pictures of 28mm figures in the trenches for comparison.

Cheers
Paul T@EWM