Polycarb can be bent cold, so you could do Pheonix’s tabs and bolts suggestion all in polycarb.
You could also use nutstrip. BBB sell some in M3 and M4 (https://shop.bristolbotbuilders.com/product/nutstrip/), or I designed some lighter 3D printed M2 ones (3D printed M2 nutstrip). I know you don’t want to get into printing yourself, but I’m sure you can find someone happy to print them for you. (I could if you’re willing to pay a few quid, but I don’t want to pressure you at all)
Another option would be to use small blocks of HDPE or tough wood, and fasten the polycarb to them with small wood screws