from what I've heard and read (i.e. 2nd hand knowledge) the Holgas are very prone to light leaks. If all you want is a box to hold film flat while you put an old folder/TLR lens on it, why not get an old folder/TLR? You could buy one or two for what you'd pay for a holga... I mean, If a mediocre lens is what you want, buy a mediocre camera, right?
I know, that takes all the fun out :-)
Anyway, check out freestyle, in their catalog they sell a wolga (sp?), a Holga with a glass lens. Might be what you want, Although I'd rather have more shutter speeds and f-stops.
Keeping in mind that I had a mediocre TLR, I'd rather buy another TLR than a Holga