I took an old lightweight standard fly rug and butchered it! I made a slit from around 6" behind the wither, to about 6" behind the cantle. This gave me enough give in the fabric to be able to lift the saddle flaps through, complete with stirrups and with complete belly protection. For me, this is the only way, as I have a small pony, so the ride on rugs don't go small enough and in any case, it's normally the belly that's the issue!! Mine is very grimy, now, so I plan to buy a new cheapy one and do a neater job of it!
I just use my normal fly rug with belly. It's quite a loose fit so am able to ensure enough wither clearance when putting the saddle on, girth does up in front of the belly flap. I wouldn't do serious schooling with it but for hacking it's fine.