They can be expensive. An alternative is the 'poor-man's' high magnification enlarging lens. That is, use a lens with a longer focal length. For example, when I'm projecting 8x10 on the wall for up to 50" I use a 'standard' 360mm enlarging lens. Realize that, just like large format photography with lenses that 'just cover' you problems arise at infinity, not close-up. So when enlarging, small magnifications are like close-up and you don't need a lens with great coverage. The more magnification you need the more it is like a camera lens at infinity, thus you can run out of sharp image circle.
Originally Posted by Old-N-Feeble
UPDATE: I saw above that you may have got a bargain priced Rodagon G 300! Good work!