Use a simple eyepiece and project the image onto a card. By simple I mean one without cemented elements. Ramsden is a good choice. Of if you only have cemented eyepieces stop down the scope to about 2" off axis and the heat shouldn't hurt a cemented eyepiece like a Kellner.

Don't risk anything more expensive however. With your f/12 scope such cheap eyepieces perform very well, just that they have a narrow field of view. Image quality is virtually the same as their expensive cousins.

Rick