P38 Seat and Zip Ties

Last year I posted about a problem I've had in the rain. Once my lightning P38 gets wet, the seat fabric
(and/or the cording holding it in place) stretches enough so the seat back rubs on the rear tire -
particularly when "pushing hard" from a dead stop - and I'm a lightweight rider. Moving the seat further
from the wheel isn't an option (the stays are frozen, I'm torch-adverse, and I kind of like it where it is).
Besides, while dry the seat never comes close to the tire - ever.

While I could have been more scientific in determining what was the real culprit, I decided to attack all
weak links simultaneously, all with suggestions from this list.

Under the seat fabric I added a grommeted "underseat" made of Phifertex
fabric<http://store.valueweb.com/servlet/poweron/Detail?no=82> which presumably doesn't
stretch when wet). In addition to keeping the seat from getting close to the wheel this also acts as a
protective barrier if the seat and tire do meet (the underseat will rub on the tire first). I've also
replaced the cording with zip ties through the grommets in both the P38 seat and the underseat.

Early results are very positive - I've ridden in the rain multiple times without any hint of a problem, and I
had problems every time it rained before.

The new problem is that some of the zip ties seem to work loose after a few rides. Maybe I used the wrong ties?
Mine are the "Commercial Electric" brand, distributed by Home Depot. I seriously doubt that I've applied
them backwards as they only seem to catch one way. The packaging makes the following claims:

- Indoor/Outdoor
- Meets Military Specification
- UV Resistant
- 75lb Tensile Strength
- Double Lock Grip (33% stronger)

They're roughly 5/32" (I believe that's 4mm) wide. I think I could go a little wider, but too wide might begin
to misshape the grommets (or simply not fit through). Can anyone point me to a better zip tie or offer