I have just taken delivery of my new torsion deluxe

I have splashed out on a torsion front pommel bag which I will keep on saddle all the time. However I really can't justify the expense of a rear saddle bag by torsion! Can anyone out there recommend a good quality bag that fits securely onto the D rings with extra straps to stop bouncing - needs to be big enough for packed lunch, waterproof mac etc - up to the £50 mark. Thanks all x