Something I was looking at recently is these bags made by Brooks;

http://www.amba-marketing.com/products.php?cid=34

Sizes and prices are:

D Shape - 16cm x10cm x5cm, £42

Glenbrook - 33x 18x 16.5cm, £48

Millbrook - 23x 18x 16.5cm, £35

Brooks are an old UK company known mainly for their bicycle saddles, but I think these bags may look OK on a Triumph?

Brooks website is http://www.brookssaddles.com/ but they've not got around to listing the bags on the site yet.