I've found the price of loudspeaker spikes far too high for the simple item that it is, and so decided to make my own from commonly available materials.
Step 1: What You Need
1. A T-nut (left in the first photo), or threaded insert (to the right on the photo). A threaded insert occupies a smaller area and is a more secure fitting, but is more difficult to work with, so I usually prefer the T-nut. Here I've use one with a 6 mm inner thread.
2. A 20 mm long 6 mm bolt with the head cut or sawn off (second photo), leaving you with a length of about 18 mm.
3. A nut which fits the bolt (to the right on the third photo).
Note: You can of course change the sizes to suit your particular speakers.
Step 2: File the Bolt
To give you spike a sharp point, you have to shape one end of the bolt with a file or grinding wheel. How sharp, depends on how you want to use it. If the speaker will be use on a solid floor, a point like in the photo will be sufficient. If, however, it has to penetrate a carpet, it will have to be filed to a much sharper point. Remember to shape the end where the bolt was separated from its head, and the thread thus already damaged. That way the "good" end of the bolt will stay undamaged.
Step 3: Drill the Hole
Drill an 8 mm hole for the T-nut in the bottom of your speaker enclosure. In this particular case it can be the same depth as the T-nut. Of course you will actually have to drill four holes for the four spikes per speaker, but we're focussing on just one, because they're all the same.
Note: It is preferable to use a drill bit for wood like the one in the photo, because it drills neater in wood than drill bits meant for metal.
Step 4: Fitting the T-nut
1. You can just fit the T-nut into the hole by tapping it in with a hammer, but safest would be to drill small guide holes for the T-nut's spikes to prevent cracking of the wood. To do this, put the T-nut in the hole (first picture), and press it down firmly until the spikes mark the wood quite clearly like in the second photo.
2. Now drill 4 guide holes of about 5 mm deep for the T-nut's spikes (I used a 1.5 mm drill bit). You can now tap the T-nut into place with a hammer (third photo), until it is seated (last photo).
Step 5: Finishing Your Speaker Spike
1. Screw the decapitated bolt into the T-nut to the correct depth (first photo) with it's sharp end pointing upwards, and then screw on the nut.
2. Tighten down the nut, and you speaker spike is finished (second photo).
3. Of course you can also paint it for a better look (last photo).
Step 6: Enjoy!
Enjoy the hopefully better sound of your newly spiked speakers. Spikes supposedly couple the speaker better to the floor, giving a more solid bass response, and also serves as feet to keep the bottom of the speaker cabinet from being damaged by the floor. If the underside of the speaker or the floor, or both, are uneven, adjustment of the spikes will also solve that problem.
On the photo you can see the spikes fitted to a Visaton open baffle speaker.