So, I have an old solid state "Matrix" branded amp that went out a long time ago. I kept the cab and trashed the the actual amp head. It was a 100 watt amp with two 8'' 8 ohm speakers. I'm sure they're not all that. But since they're there, I was thinking I could use it as an extension cab for my peavey classic 30.

Would this work out? I know it may not sound great at all but I'm just interested in giving this prop some use. I also plan to build a 1x12 cab for my classic 30 later on(another thread in a week or so).

If it would work, what do I need to do as far as wiring? I know I need a 1/4 jack but how would one go about rigging it up?
Mojotone for parts. Google the rest- just keep in mind what your amp selector has- 8/16 or 4/8/16 ? Or no ohm selector?

If your amp has no selector then you must keep it at the rating of your stock speaker. Most are 16ohms. (Singe 12inch speaker)

If you wire your two 8 inch speaks (which are at 8ohms) in parallel then you will be 4 ohms.
If you wire them in series then it will be 16 ohms.

You'd need a jack and speaker wire.

If you do it cheap it could be a fun little project. Will sound like shit still.
Looks like the Classic 30 is 8/16ohms so you'd need to wire the speakers in series. Make sure you wire them in phase (-ve of one speaker to +ve of the next) or you'll get some noise cancellation. Other than that it should be a nice easy build. Enjoy.