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Malcolm,
You you could cut a 4" hole in the lid, off-axis, and turn it into a nice 4" f/15 scope (same focal length as the 12" was). Would be even better than a refractor as its achromatic.
BUT you can't make two, three or even four 4" diameter holes and expect it to perform as an 8" - you might match the light grasp but the resolution will still be that of a 4", and the Airy disk is going to look pretty ugly.
A mask with a single 8" diameter hole is another matter, it will made your scope work as an 8" f/7.5, and the result could be surprisingly good since the worst part of most mirror figures is the outer inch - masking it off often helps the resolution.
Years ago I some fun with assorted masks on a C8, mostly made of thin card with holes cut with a sharp blade - and Andrew James used the idea to create some interesting diffraction patterns that assisted with determining the separation of close double stars.
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