As stated above just squeeze them in. Although some 1/10 4wd front tires come with rear foams you can cut those down.

Also, some racers trim the foam down to the with of the actual tire before installing. But squeezing in what it comes with will create a firmer tire.

And then, you can always trim the edges inner and outer to curve to the edge of the tyre. Also avoids the tyre glue accidentally gluing the foam to the rim when glueing the tyre on.