You can buy plugs in any possible shape, size and color. However you may have you own ideas or wishes, like a possibility to lock the plug in place, to make it long or thick, or curved to stimulate the prostate while allowing the muscles of the anus and bowel movements to slide it in and out. I tried to implement the latter idea.
In order to make a lockable butt-plug you need the following:
Roll a condom over the chain
Fill the condom with silicone sealant. Begin from the tip and work towards the opening
Use your imagination. I wanted to build a very long, curved and flexible plug which could potentianlly penetrate the sigmoid (see below). Thick wire can be used to keep the desired shape until the sealant cures. Exposed to air silicone cures within 72 hours, but since it's covered with latex it takes a couple of weeks.
This is the result. More rigid than I expected but it gives very interesting sensations. Next time I will try to make it thinner at the tip (last ~15cm) to make it more flexible and easier to insert.
The plug should be fully inserted with only the last couple of chain links outside. But as you have noticed, the DIY plug does not have a flare base. Hence, to prevent "swallowing" thread a rope, chain or padlock through the chain links and connect the plug to, for example, a crotch rope (see the Basic Techniques section).
Using a condom (shown) over the plug and lots of lube (not shown) is highly recommended. And, of course, be carefull and do not force the plug into you body. Also keep in mind the natural curvature of the colons (see below).
Some theory. This is what the female and male anatomy looks like:
Also note the average dimensions. Though, you can try and measure your own rectum ;-)
Some useful links: