Fuji Rod Caps fit a wide range of tenkara, keiryu and seiryu rods. Use the PTC-10 for the thinnest rods, KTC-12 for rods that are a bit thicker, the KTC-16 for thicker rods and the KTC-20 for most 6+m keiryu rods. The number tells the diameter in millimeters. Measure the diameter at the end of the grip section to decide which rod cap to get.
They make an excellent back up rod cap for just about any rod you have and some people prefer them to the more standard tip plugs. They may be a bit harder to store somewhere when you are fishing, but they are also a lot harder to lose.
Perhaps the best thing about them (other than being an effective back up for a variety of rods) is that they are a lot more secure than most tip plugs. For several of the first tenkara rods I got (before I started importing rods from Japan) the tip plug wasn't quite tight enough and would fall out at an inopportune time or even in the rod sock. Luckily I never broke a rod because the tip fell out, but that is a definite possibility if the tip plug is too loose. These are never too loose - at least if you get the right size.
The KTC-12 comes standard with a number of the rods I carry, including the Suntech GMR Special and Suntech GM Suikei Keiryu Special 39, both shown above.
They really are a good alternative - I wouldn't them "universal" though, because they really aren't. They come in sizes and you will want to get the right size. The KTC-12 is for rods with a diameter (at the end of the grip section) of 12mm or less. The KTC-16 is for rods with a diameter of between 12 and 16mm. The KTC-20 is for rods with a diameter between 16 and 20mm.
For rods that are less than 10mm at the end of the grip section, the PTC-10 would fit better than the KTC-12. The PTC-10 will fit the Suntech Kurenai HM30R, although it is a bit larger than necessary. It fits the Kurenai HM33R perfectly. The PTC-10 will fit the Nissin Pro Square and Pro Spec 320 rods. It will fit the Royal Stage and Zerosum 320 6:4 rods but not the 320 7:3 rods, which would require the KTC-12.