Use a hammer drill to drill two holes (preferably) in the live rock, one towards each end. Make sure you drill for strength. Place the liverock on eggcrate you sized to suit your needs and push the ziptie through each hole, through the eggcrate, and attach. This is where your creativity comes into play. You can use foam in the cracks to fill gaps so that you cannot see eggcrate and keep any rocks bound together. Here is an example from a larger tank: My DIY Rock/Foam Reef - Reef Central Online Community
I would use more rock than foam, moreso as a filler. I would use Great Stuff: Pond and Stone.