Bob,

A cheap & cheerful method :

Set the focus of the scope to its minimum value, then cover the objective lens with a piece of aluminium foil, and poke a small hole in the foil.

Start with a very small diameter hole, then work upwards - it is a compromise between a brighter image and maximum depth of focus - just like any aperture.

The hole in the foil does not need to be in the centre of the field.

This technique of using a sub-aperture mask has been used quite a lot with astronomical telescopes.

Have fun & a good weekend

Best regards

Russ