Golang: how to use interface{} type to insert a value into the middle of a slice?

I'm having difficulty getting my head around the usage of interface{} types with Go.

In this instance I have a function which inserts a value into the middle of a slice somewhere. It looks like this:

type mystruct {
    a, b, c int
}

func insert(ar []mystruct, val mystruct, i int) []mystruct {
    l := len(ar)
    if l == cap(ar) {
        tmp := make([]mystruct, l + 1, (l * 2) + 1)
        copy(tmp, ar[0:i])
        copy(tmp[i+1:], ar[i:])
        ar = tmp
    } else {
        ar = ar[0:l+1]
        copy(ar[i+1:], ar[i:])
    }
    ar[i] = val

    return ar
}

I'd like for that function to be able to accept interface{} for both ar and val so that I could pass it a slice and value of any type and it would perform the insert without complaint.

From what I have read so far I believe this should be done with the reflect package. I have read the Rules of Reflection and various tutorials on reflection and interface{}, but I'm not sure how to approach this. I'd like to minimize overhead as much as possible, I understand reflection is going to slow the code down a bit but what is the most efficient way of doing this?

Thanks!