If you have the FRP seat with the vent holes slotted into it from Actionbent, do include the slots in your template. That will help most of all for ventalation. I would also consider a built in neck rest in the pattern.
Shaman,

I think there's an error. Here's a pic I took from the seat I got with the AB HiRacer. The HiRacer spec from AB's web site reads like:

"Seat: UltraLite FRP (Fiberglass-Reinforced-Plastic) w/ Headrest and velcro-detachable pad "

I got the HiRacer a little more than a month ago. If I then look at the AB web site in the Seats/Pads section, clearly the ultralight FRP seat had those vent holes you're mentionning.

So it seems I did not get the right seat. Hmmm...