Why not give one of these new Pacific aerials a try.
I am going for the 1 meter one P6182 and they do a variety of bases that will suit an a-frame
They do 2 versions one with the wire attached into the aerial (P6035) the other where wire runs into the base and the aerial can be unscrewed from the base (P6182). I like this idea as it would make it very easy to change if it breaks in future. and you could tie wrap a spare antenna to the underside of the A-frame.
Marine Megastore seem to have it all in stock although their part numbering is different so if you go to www.smgeurope.com
they are the UK distributor and have better info from which you can get a UK part number to go to marine megastore with