Sam, with pony trucks I just add plenty of weight and then leave them to their own devices. The loco is ballasted well, and I make sure the centre of gravity is acting over the drivers. The pony truck is made out of a strip of .018” brass, with a 6BA nut at the back on top. A 6BA screw passes up through a frame stretcher, and this forms the pivot for the truck, and it is a sloppy fit in the stretcher, so the truck can rock fore and aft, and tilt side to side. The axle runs in a brass tube soldered under the strip. The strip is then sandwiched, on top with a length of 20mm x 5mm steel strip (from B&Q) for the weight, and part way underneath with a bit of glass fibre copper clad. The sandwich is held together with two 8BA nuts and bolts. The copper clad can take wires back to the motor with some scraperpickups using phosphor bronze strip (Slaters) as I reckon small tankies need the pickups. There’s a square bracket comes down from the body at the front to limit sideways swing, I suppose a proper craftsman would try some sprung side control, I’ve left it. Hope you can pick this out on the photo, the main pieces have been painted black.