I have been fooling around with helper belts for a couple of
seasons ...

The following scheme works pretty well ... I tested at the recent
Region 2 contest ...

My wife sewed a couple of loops in 2 " webbing ... the loops are
slipped over the shoulder at the buckle ... the free end passes
through a 2" D-Ring which is secured at at the lap belt attachment
point ... i.e. you remove the lap belt and then re-attach with the D-
Ring included in the loop that circles the the side attachment 'bar'.

The free end of the helper webbing then pass up into the same spring
loaded buckle holding the shoulder straps ... and yep, this works
fine ... no slipping on either belt ... and both are easy to
adjust ...

While this worked ok, finding the D-ring next to your thigh while you
were sitting in the ship was a pain ... so the next approach simply
attached the D-Ring to the free end of the lap belt using a 2"
slider ...

I don't have a photo, but this worked great ... as you bounced on the
ridge, the tension on the shoulder straps kept the laps pulled
tight ... the hold down pressure is quite good and I actually had to
loosen the rig from the pre-takeoff tension as I had it cinched down
tight anticipating it would loosen up ... it never did.

So, is it as good as a five-point ... well, you do have two extra
belts to mess with ... and the 5-belt system is rock solid ... but it
is darn close ... plus it is portable/reversible ... and there is no
restricting already limited crotch real estate.

BTW, when you release the buckle ... the loops pull through the D-
rings ... so you can get out gracefully ... or jump ...

I have a bunch of extra 2" webbing (maroon and blue), D-rings and
sliders ... you can have these for cost + postage (~$10) .... the
webbing has to have the loops sewn ... i.e. I'm not sure I could
impose on my wife ...

Also, looking at other options, I think you could probably just loop
the belts through your crotch and then up to the shoulder buckle to
create an anti-submarine system ... I have not really explored this as
I was just looking for additional hold-down force ... KK

http://sailplane-racing.org/Belts/KKRidgeRope.jpg