@Carl Hall I used:
Polycarbonate sheet. I got it from Amazon but you may get it from B&Q or the like. Or even an old "For Sale" sign, I believe they use it.
Sticky backed felt. Cheap from Amazon.
Stiff horticultural wire. Cut up wire coat hanger would do I suppose. Bent to form a U shape about 6 cm long for rigidity.
The only tools needed are a craft knife to trim the sheet and cut the felt ( A saw of some sort helps when cutting the sheet across the chanels).
A straight edge to guide the cut, and a flat cutting surface.
Here's some shonky 'phone pics to give you an idea...Any questions, shout up.