The following photos will give you an idea of a compact overflow tank that you can fabricate from PVC pipe. It mounts next to radiator on the passenger side of the car, using the bolts that hold radiator in place.
The main tank is made from 3″ diameter and 6″ long PVC pipe with two caps cemented to each end. Another 2″ diameter PVC pipe is used for the spout, with a cap fabricated to fit insideor outside diameter. My cap is a vented aluminum, with “WATER” engraved on top. A hole is drilled so the overflow line can be inserted into tank. This line should reach the bottom of your tank.
The mounting plate is 13 3/4″ long and 1″ wide. Two 21/64″ holes are drilled on 12″ center. Check you bolt location to make sure they are 12″ apart. The plate has a bend on both ends so the tank will locate back toward the engine for clearance. A spacer or washers are put between the plate and the radiator bracket so it does not interfere with the bent portion on the upper bracket.
As the photos show, I have attached the tank to the mounting plate with two hose clamps and painted the PVC black.