Good grief Pierre...why hadn't I thought of that??? I spent last year restoring a '74 Beetle and no telling how many times I moved that plastic reservoir around the shop....and never considered using one to replace the leaky reservoir in the RV-6.
Several clear reservoirs are available in Aircraft Spruce Catalogs.
My concern is flammable liquids in the firewall fwd area that could melt and turn a small fire into an inferno. Hazard level depends on the brake fluids used also.