My old 29 had little tracks for the curtains to run on, in the end replaced this with bog standard curtain wire using the hooks /eyes that came with them. You can place the eyes to go with curvature of the hull, they make a nice fit and easy to take down/clean etc. One wire above, one wire below windows.
Sorry for being facetious. I have little wooden blocks glassed onto the inside of the coachroof that I can screw brass eyelets into; these take the hooks on the end of net curtain wires, threaded through the seams in the curtains. A headlining may make this more difficult.
Ive successfully used plastic oval shaped mains wire conduit as a neat curtain pole on spans up to 18 inches (0.457m). The conduit (about 12x6mm section) fits in a pocket at the top of the curtain and is held in place between a pair of small stainless lacing hooks (Z shaped things about 1inch long held with two screws). The plastic conduit is flexed to fit between the hooks which are fixed horizontally with their points facing each other ie. pole held under compression with the hook point inside the end of the conduit tube. (how do you get grafix on this thing?)
The pole is easy to fit and take off and is neat enough not to need a pelmet to hide it. Make the pole wider than the curtain so you can open them. Alternatively the curtain can be lifted up and held behind the pole.
A bigger span needs larger conduit held in place with plastic pipe clips either the push on "C" type or the ring type with hinged flap.
A friend recently bought a sweet little 20 ft keel boat. I was really impressed until I noticed that on both sides of the cabin sides around the windows was a line of screw tips breaking through the gel coat . The fellow must have drilled and tapped or used self tapper screws to hold the curtains up. I am not suggesting for a moment you could be so stupid yet here was someone who was. No wonder he sold the boat. So as suggested bond (glue) blocks of wood to fix into.
I once had an MG Spring that had the same problem. I used small plywood blocks which were Araldite-ed onto the GRP cabin sides, and then bought some nice little curtain rails from a caravan shop. The advantage was the curtains slid with ease on them, they bent to fit the shape of the coachroof and you didn't get a hook just in the wrong place.
If none of the other suggestions work better for you, then I'd suggest the following:-
Stick on some self-adhesive velcro loops onto the headlining if solid (otherwise sew non adhesive stuff) and sew a small amount of velcro hooks onto the curtain. Then just press the curtain into position. Not pretty, but it would work. I've had to adopt this solution for a bench cushion that would never stay put and always landed up on the sole. It has worked for a season now.
Make sure the adhesive backed loops have a much greater area than the hooks, that way it's unlikely you'll pull the self adhesive stuff off (been there, done that!).