I have and use a Peak 1, its amazing and I would argue one of the best, if not the best grain focuser I have ever used. It is a bit pricey but used ones pop up from time to time.
If you would like a bit more distance from the eyepiece (more eye relief) then you can try the magnasight, it has a very large viewing area but the image is not as magnified. Barry Thronton states and illustrates in his book the edge of darkness that he uses a modified mangasight with a drop in lens for increase magnification.
If you want lots of manification, you can get a microsight I think its like 25x. Both manga and microsights are very affordable.