For example,

    package main
    
    import (
    	"fmt"
    	"math/rand"
    	"time"
    )
    
    func padWithRandomBytes(b []byte) []byte {
    	s := 5 + rand.Intn(5+1)
    	e := 5 + rand.Intn(5+1)
    	slice := make([]byte, s+len(b)+e)
    	copy(slice[s:], b)
    	return slice
    }
    
    func main() {
    	rand.Seed(time.Now().UnixNano())
    	b := []byte{1, 2, 3}
    	fmt.Println(len(b), b)
    	s := padWithRandomBytes(b)
    	fmt.Println(len(s), s)
    }

Output (random):

    3 [1 2 3]
    20 [0 0 0 0 0 0 0 0 1 2 3 0 0 0 0 0 0 0 0 0]

    3 [1 2 3]
    15 [0 0 0 0 0 0 1 2 3 0 0 0 0 0 0]