Hi @LegoCaltrops, only me!
I think basically what you're looking for is a screen that will affix to the internal uPVC frame around the top openers. How it attaches depends on how much you want to spend but it sounds like you'd rather they're not too expensive.
I was the same, just wanted to pay as little as necessary for something that would do the trick. I needed to spend more for one screen which is our exit window upstairs and is a tilt & turn window that opens inward, so it has to be fitted externally. The rest of my windows are top openers so for only the ones that I want to be able to keep open I bought a kit that contains fly mesh & some velcro-type fastening, which you can cut to size. The mesh is attached to the velcro strip all the way around when the window is open and then you just unattach it to reach the handle, to fasten the window.
I bought my screens from a company called New Blinds and got this kit for the top openers…
newblinds.co.uk/velcro-mesh-fly-screens-flystop-plus-150x180cm-velcro-mesh-window-fly-screen
There are 2 different sizes in this plus version & there is also a cheaper version. After measuring I also decided to get some extra velcro tape which is available in their window screen accessories section.
If you're happy to spend a bit more for something more sturdy there are magnetic fastening screens, which can also be cut to size, but I think one kit would only do one window, whereas with the velcro ones you can do multiple windows from one kit depending on window size.
Magnetic screens here newblinds.co.uk/magnetic-fly-screen They would just lift and rest on the handle when the window is shut, much like the velcro ones.
If you wanted to cover your full opening window you could use something like this - newblinds.co.uk/velcro-mesh-fly-screens-flystop-skylight-130x150cm-velcro-mesh-skylight
Similar products from a different company here
www.flyscreenqueen.co.uk/flyscreen-windows/simple-window-kits.html This uses polyester fabric rather than fly screen mesh, would still work though.
www.flyscreenqueen.co.uk/flyscreen-windows/magnetic-flyscreen-windows.html
Fly Screen Queen have short videos too, so you can see both these types of screens in action.
Another alternative is this www.easy-flyscreens.co.uk I intend to create my own version of these eventually but just got the cheaper option to do for now.
Ok, sorry it's such a long post but I hope it is helpful to you & you manage to find something suitable.