All engine faults/repairs/maintenance Engine bay seal
Replacing Engine tinware seal - aircooled
Its important with the aircooled engine to have an adequate seal between the top half of engine bay to reduce heat generated by the exhaust. Likewise there should be no leaks in the ducting. Do not confuse this with the seal directly under the engine lid which has a square profile.
Given that the proper tinware seal is around £100 in some places (and the T2 seal much smaller and different in profile), here is an alternative for around £25 (seal ETS1395):
Sealsdirect.co.uk make this stuff (check for current prices) http://www.sealsdirect.co.uk/bbCMS/shopping.asp?intDepartmentId=68#23 (last seal on page)
Whilst different from the original, it will do the job. Next photo is the original alongside the one from seals direct (thanks to member 'A Kirk' for photos. You will need 4 metres to go around the engine lip.
And here it is installed