I use the putty stuff on mine, works great. To do it right though you would need to also seal the gauges around where they mount to the binnacle, air will flow by them otherwise.

they make a putty which never drys.  It's used to pack around loose window frames in the winter. I'll find the name tomorrow at work and add it here.  It's a grey putty. It works great and similar to the OEM stuff.  You take a few pieces and roll it out thin about the thickness of a pencil and then press it onto the binacle, then fit the piece up to the firewall. Seals well, moisture and air.

You could use any of the thin foam rubber tapes used for weatherproofing homes.  If that is not what you had in mind, strip butyl caulk (also available from the home center) could be used.  My preference with the old plastic is to use the foam tape since it compresses easily and won't glue things together.

What is the sealant used between the cowl (holds the center spedometer) and the body of the car (firewall)?  What is available today that does the same function?