the universe/classics onslaught is actually an ultra and might seem out of scale with the tiny deluxes..
might I suggest Transformers Cybertron "cybertron defense scattershot.
he may be more in scale and he might do a great job filling in for a voyager class onslaught.
or there is transformers energon Barricade
he is a little smaller but he matches the G1 onslaught the best.