I think this is the picture you are looking for.



I own the double breasted version of the doublet. My concern would be twofold for the conversion. First as Captain said the material looks quite a bit thinner than the material used for a real doublet so it will get sloppy looking when you wear it quite quickly. You might be able to help this with a good dose of starch when ironing the jacket. I'm not a tailor, just a suggestion.

Second, a doublet is really a fitted garment. I'm sure they sell off-the-rack but I don't know anyone who has done that. Maybe someone here has done it and can give you advice. They tend to be made to measure. While I know you want a DIY project you might want to consider the help of a tailor for part of it. I think you can easily handle the shortening and addition of the belt loops, but you might want to have a tailor help "fit" the jacket for you.

I'm sure that might effect your budget, but in the long run it would probably be worth it.