If you want to auto cross or race your Tiger, or just want a 4-point seat belt for your car, you have a very nice option available. The Schroth 4-point seat belt.
It has several very nice features:
a. It is a 4 point seat belt approved by DOT and FIA.
b. It does not need the 5th belt, or crotch belt, as it has an anti-submarine feature in the lap design.
c. Unlike most 4 or 5-point seat belts, this belt has a normal buckle/release feature, which is similar to your car, and not the cumbersome latching system used on racing belts.
d. Schroth provides an additional option, which allows a retracting mechanism. This allows you to reach for the radio under normal driving conditions, but turn it off for auto crossing or racing. All race approved.
I found the belt when looking for seats, and liked the quality, design features, and the ability to meet various DOT and racing requirements. This is a very common option add-on on the BMW cars and the brand is used extensively on the F1 circuit.
The installation is relatively easy. Here are the steps.
1. The lap portion of the belts uses the standard reinforced holes, already in the Tiger, or Alpine. In fact, the bolts supplied, through German, screw right into the bolt holes and are not metric.
2. The 3rd mounting point can use the built in mounting hole in the rear inner fender well area, if you are not using the retractor mechanism.
If you are using the retractor mechanism, then you need to drill a new hole, for the mounting bracket. A plate is provided, which when located, can be tack welded to the inner fender well, like the stock mounting bracket.
The photo show that you can mount the mounting bolt rearward, and still not have to cut the MKI metal convertible cover. This is not a problem with later cars.
Special Note If you have a MKI, with the hard top metal covers, this will not allow the covers to swing forward all the way. I have a MKI, and I just use 3 knurled bolts, which I remove, and take the metal cover out of the car before erecting the top.