You'd have to use screws, I think, something like lag screws and possibly put them in a frame specifically designed for that purpose.
However, if they are the panels I'm thinking of, I believe you're wasting your time. I think they are rated at 15 Watts each, for a total of 45 Watts. In practice, if this is the case, you will generally see about half that output or a total of no more than 22 Watts on a good day. At a charging voltage of 13.5V, this only delivers 1.63 Amps to your battery. That's essentially a "trickle charge", and will not do much for you while camping.
I tried using 4 of the 15 Watt panels when I first got my HiLo and found them to be more trouble than they were worth. I had them on a stand that I could set up outside and orient perpendicular to the sun. They now live in the landfill.
If you want to dry camp and charge your battery with solar, you need at least two 100 Watt panels. I have three. They are lightweight, flexible and are held on the roof by EternaBond tape. I have a thread on my installation in this forum if you want to look for it.
- Jack