In my opinion I don’t think there is really a specific brand of half pad that is made for horses that are built downhill. Half pads are used to help your saddle fit better to horse, so you might need a riser pad to make the saddle flat, a thinline pad with shims, a sheepskin pad to help reduce pressure points and make the saddles more comfortable, etc. There are so many different half pads that have so many different purposes. If you have a local tack shop near you, you can go and ask the people working there what they think will be the best fit for your horse, a great place to go is to dover. Usually they have very knowledgeable people that will happily help you. You can also have a saddle fitter come out and help find you the best fit. I hope this was helpful!
Thin line, you can customize size,padding ect. Totally recommend